Don't override previous_tab and indent_backwards shortcuts on OSX

Prior to this commit, the shortcuts for these `paredit_wrap_square` and `paredit_wrap_curly` were overriding the OS X shortcuts for the `previous_tab` and `indent_backwards` commands built into Sublime.

The addition of `ctrl+` definitely makes these two shortcuts more cumbersome, but I think it's best not to override two of the most used built-in Sublime shortcuts.

Happy to change this to another set shortcut if you have a better idea :dancer: