Add Chruby Integration

I ran into issues when switching my ruby version when already in a gem environment with ohmygems. The most direct and manual solution is to call `ohmygems reset` before switching the ruby version. However, some of my projects have a `.ruby-version` file and I have configured chruby to switch my ruby version automatically. When this happens the ENV variables used by chruby and ohmygems to contain out-of-date or incorrect values. So this script attempts to correct that by reverting and reseting `ohmygems` before and after each `chruby` call.