Fix install script to be runnable for updating
Fix tmux.conf so tmux works both in OSX and non-OSX systems

The tmux-MacOS-pasteboard patch that allows to copy/paste from within
a tmux session in OSX should not be used in non-OSX systems.  This
commit fixes this by gracefully failing if the patch is not present.

More info here:
https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard/issues/8