Maximizing parity between development and production environments is a best practice for minimizing surprises at deployment time. The version of language VM you're using is no exception. One approach to this is to specify it using the same dependency management tool used to specify the versions of libraries your app uses. Clojure uses this technique with Leinigen, Scala with SBT, and Node.js with NPM. In each case, Heroku reads the dependency file during slug compile and uses the version of the language that you specify.
Today, we're pleased to announce that we've added support for specifying a Ruby version to Gem Bundler, the dependency management tool for Ruby. This will...