如标题所示，gem 'coffee-rails'从我的Gemfile中删除后，我无法在Heroku上完成构建。
下面是错误消息以及之前的几行，表明它正在删除coffee-script。
remote: Removing coffee-script (2.4.1)
remote: Removing coffee-script-source (1.12.2)
remote: Removing coffee-rails (4.2.2)
remote: The latest bundler is 2.0.0.pre.3, but you are currently running 1.15.2.
remote: To update, run `gem install bundler --pre`
remote: -----> Installing node-v8.10.0-linux-x64
remote: -----> Detecting rake tasks
remote: -----> Preparing app for Rails asset pipeline
remote: Running: rake assets:precompile
remote: Yarn executable was not detected in the system.
remote: Download Yarn at https://yarnpkg.com/en/docs/install
remote: rake aborted!
remote: LoadError: cannot load such …