Fix with rbenv

This package wasn't working for me out of the box using zsh + rbenv. I added an option to set the shell. I also found that there was a discrepancy between the options namespace being used and the package namespace, so I changed instances of `atom-rspec` to just `rspec`. Finally I added keybindings for vim-mode that match the defaults used in [rails.vim](https://github.com/tpope/vim-rails).

Let me know if you want separate PRS or need tests.