set `RUBYLIB` env var

[kellyredding]

bundler is moving to using this when it installs its stuff: http://bundler.io/v1.5/whats_new.html#version15

We should probably be setting it.

http://stackoverflow.com/questions/17300695/what-does-export-rubylib-path-gem-lib-do

[kellyredding]

(notes to self)

I want to wait until Bundler releases v1.5 and see how they use it before moving forward with this.