Separate gem directories for different Rubinius versions

[mvz]

It seems gems for all versions of Rubinius are put in ~/.gem/rbx/2.1.0. Would it make sense to separate them out a bit more?

[postmodern]

In e9e81df we added support for detecting RUBY_ENGINE_VERSION and use that in the GEM_HOME (falling back to RUBY_VERSION if RUBY_ENGINE_VERSION isn't present). I just need to work on releasing 0.4.0.