Set the current working directory of the process

rbenv uses the current working directory to set the ruby version (https://github.com/sstephenson/rbenv#choosing-the-ruby-version). Ruby Markers should set the current directory of the subprocess to that of the current view (prevents many many headaches!) or default to the user's home

Note: I'm not typically a Python coder or Sublime plugin author but the code works reliably for me (ST2, OS X). Not sure how to make this work correctly on Windows