The Ruby platform uses ruby 2.2.3 by default and get your dependencies from Gemfile.

You can define what ruby version you want to with two ways:

* set ruby version in ``Gemfile``:
    $ cat Gemfile
    ...
    ruby 2.2
    ...

* ``.ruby-version`` file:
    $ cat .ruby-version
    2.2

The platform downloads ruby from a remote uri and you can define an environment variable RUBY_REPO to customize where from it will be downloaded. The platform will download ruby from: $RUBY_REPO/ruby-$RUBY_VERSION.tgz, where RUBY_VERSION is the ruby version figured out by the last step.

You can configure a custom source to be used for your gems using the environment variable GEM_SOURCE, this will be passed to the gem install --source argument.