Adding rbenv support

Add logic to get the rbenv command prefix, and use prefix in all ruby commands
To use, you'll have to also have rbenv.el

It should probably be noted that in theory, as long as your PATH is set correctly(like, for example, using exec-path-from-shell), this shouldn't be necessary, as rbenv will add itself to the front of PATH. It might be that the better option is:
```elisp
(when (and (boundp 'rbenv-binary-paths)
           (not (string-match "shims" (getenv "PATH"))))
    (message "Your path isn't set correctly to use rbenv in emacs. You should install `exec-path-from-shell'"))
```

or something similar.