Using environment variable RUBY_VERSION if it exists, and failing back t...

...o GEM_HOME.

Reason being, if your app is using ruby 2.1.2 (and it may be this way for others)
and you run 'bundle install --deployment', you'll find your gems under
bundle/gems/ruby/2.1.0, and this gem thinks your ruby version if 2.1.0, NOT 2.1.2,
which causes obvious issues during deployments.