WIP: Trying to fix issue #154, Finding custom rbenv_path

NOTE: This commit is a Work In Progress. Would like to get feedback from contributors/authors

Was going through issue #154 and viewed commit https://github.com/maltize/sublime-text-2-ruby-tests/commit/ed6d6a3. The code I created below doesn't work and what works is that I just pass the string `'usr/local/bin/rbenv'` to the rbenv_cmd variable. 

Have tested the below code on the Python REPL and saw that it returns the correct path. `os.popen('which rbenv').read().split('\n')[0]` returns `'usr/local/bin/rbenv'`. When I tested the code manually from ST2, it didn't work though. I hardly code in Python(only 2nd time for me, I think) so there might be something wrong with my code. 

By the way, I get the description of the which() function when I run `which rbenv` from the terminal but I get the correct output when run via Python REPL.