Remove RUBY_VERSION env var support

The docs says it was going to be removed entirely in August 2012.

It is now Sept 2013. About time to remove it.
Merge pull request #144 from heroku/remove_ruby_version_env_var

Remove RUBY_VERSION env var support
set indy to false. Fixes #145.
store the rails 4 asset cache to improve asset pipeline performance
Merge pull request #146 from hone/jruby-passenger

set indy to false. Fixes #145.
Move logic to testable class
Merge pull request #147 from hone/cache_assets

store the rails 4 asset cache to improve asset pipeline performance
restrict build rubies to mri 1.9.2 and 1.8.7
Merge branch 'master' into run-bower-develop