部署Elovation乒乓ELO系统到Heroku时Ruby应用编译失败如何解决

我打算使用elovation开源项目搭建一套适用于办公室乒乓球赛事的趣味ELO积分系统。
找到这个项目时我非常开心，本以为部署会很简单，结果遇到了报错问题。
我尝试通过谷歌搜索解决方案，但所有结果都涉及命令行操作、编辑Gemfile、执行bundle install命令这类操作来修复。我完全搞不懂怎么按照README文件说明的流程完成这些操作，现在非常崩溃。
安装表单中有一个RAILS_ENV变量设置为"production"，我不知道是否需要修改，也不知道要改成什么值。我接触科技产品不算完全的新手，但我觉得我可能漏了什么非常简单的步骤。如果能获得帮助我会非常感谢，谢谢！

首次报错日志 #

-----> Building on the Heroku-20 stack
-----> Determining which buildpack to use for this app
-----> Ruby app detected
-----> Installing bundler 1.17.3
-----> Removing BUNDLED WITH version in the Gemfile.lock
-----> Compiling Ruby/Rails
       Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-20/ruby-2.4.0.tgz -s -o - | tar zxf - ' failed on attempt 1 of 3.
       Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-20/ruby-2.4.0.tgz -s -o - | tar zxf - ' failed on attempt 2 of 3.
 !
 !     The Ruby version you are trying to install does not exist on this stack.
 !     
 !     You are trying to install ruby-2.4.0 on heroku-20.
 !     
 !     Ruby ruby-2.4.0 is present on the following stacks:
 !     
 !     - heroku-18
 !     
 !     Heroku recommends you use the latest supported Ruby version listed here:
 !     https://devcenter.heroku.com/articles/ruby-support#supported-runtimes
 !     
 !     For more information on syntax for declaring a Ruby version see:
 !     https://devcenter.heroku.com/articles/ruby-versions
 !
 !     Push rejected, failed to compile Ruby app.
 !     Push failed

补充信息 #

补充1：当前操作页面截图 #

补充2：降级Heroku stack后操作日志 #

我尝试了网友给出的方案：

正如报错所示，项目配置使用的旧版Ruby不受当前Heroku stack支持。你可以将dyno降级为旧版heroku-18 stack，或是升级elovation使用的Ruby版本。降级到heroku-18可能是最简单的方案，但请注意elovation已停止维护，多个依赖存在安全漏洞。

操作后仍然遇到了几乎一样的报错，完整操作日志如下：

Last login: Sat Nov 20 07:26:34 on ttys000
jakejohnson@Jakes-MacBook-Pro ~ % heroku login
heroku: Press any key to open up the browser to login or q to exit: 
Opening browser to https://cli-auth.heroku.com/auth/cli/browser/ce7a8998-d5dd-4406-acd5-8f086d1f5e31?requestor=SFMyNTY.g2gDbQAAAA43My4xNjMuMTQ5LjIzN24GAGiTXD19AWIAAVGA.7u7MiKZ6EBv_y9C1X9aom-tXUK8itfOM-_EJEzD8moU
Logging in... done
Logged in as mynameisjacobj@gmail.com
jakejohnson@Jakes-MacBook-Pro ~ % heroku create --stack heroku-18
Creating app... done, ⬢ floating-reaches-04486, stack is heroku-18
https://floating-reaches-04486.herokuapp.com/ | https://git.heroku.com/floating-reaches-04486.git
jakejohnson@Jakes-MacBook-Pro ~ % cd ~/Desktop/nextphasepong
jakejohnson@Jakes-MacBook-Pro nextphasepong % git init
Reinitialized existing Git repository in /Users/jakejohnson/Desktop/nextphasepong/.git/
jakejohnson@Jakes-MacBook-Pro nextphasepong % heroku git:remote -a nextphasepong
set git remote heroku to https://git.heroku.com/nextphasepong.git
jakejohnson@Jakes-MacBook-Pro nextphasepong % git add .          
jakejohnson@Jakes-MacBook-Pro nextphasepong % git commit -am "work dammit"
On branch master
nothing to commit, working tree clean
jakejohnson@Jakes-MacBook-Pro nextphasepong % git push heroku master
Enumerating objects: 180, done.
Counting objects: 100% (180/180), done.
Delta compression using up to 12 threads
Compressing objects: 100% (164/164), done.
Writing objects: 100% (180/180), 68.05 KiB | 3.58 MiB/s, done.
Total 180 (delta 11), reused 0 (delta 0), pack-reused 0
remote: Compressing source files... done.
remote: Building source:
remote: 
remote: -----> Building on the Heroku-20 stack
remote: -----> Determining which buildpack to use for this app
remote: -----> Ruby app detected
remote: -----> Installing bundler 1.17.3
remote: -----> Removing BUNDLED WITH version in the Gemfile.lock
remote: -----> Compiling Ruby/Rails
remote:        Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-20/ruby-2.4.0.tgz -s -o - | tar zxf - ' failed on attempt 1 of 3.
remote:        Command: 'set -o pipefail; curl -L --fail --retry 5 --retry-delay 1 --connect-timeout 3 --max-time 30 https://s3-external-1.amazonaws.com/heroku-buildpack-ruby/heroku-20/ruby-2.4.0.tgz -s -o - | tar zxf - ' failed on attempt 2 of 3.
remote: 
remote:  !
remote:  !     The Ruby version you are trying to install does not exist on this stack.
remote:  !     
remote:  !     You are trying to install ruby-2.4.0 on heroku-20.
remote:  !     
remote:  !     Ruby ruby-2.4.0 is present on the following stacks:
remote:  !     
remote:  !     - heroku-18
remote:  !     
remote:  !     Heroku recommends you use the latest supported Ruby version listed here:
remote:  !     https://devcenter.heroku.com/articles/ruby-support#supported-runtimes
remote:  !     
remote:  !     For more information on syntax for declaring a Ruby version see:
remote:  !     https://devcenter.heroku.com/articles/ruby-versions
remote:  !
remote:  !     Push rejected, failed to compile Ruby app.
remote: 
remote:  !     Push failed
remote: Verifying deploy...
remote: 
remote: !   Push rejected to nextphasepong.
remote: 
To https://git.heroku.com/nextphasepong.git
 ! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'https://git.heroku.com/nextphasepong.git'
jakejohnson@Jakes-MacBook-Pro nextphasepong % 

补充3：诉求 #

我已经尝试了数个小时仍未解决，希望可以获得详细的分步部署指引，万分感谢。

内容的提问来源于stack exchange，提问作者Jacob Johnson