tmux: Fix LSOpenURLsWithRole() error on OS X

After upgrading to Yosemite, I started getting the following error when
trying to `open` any file within tmux:

> LSOpenURLsWithRole() failed with error -10810 for the file
> /path/to/file

Installing reattach-to-user-namespace via homebrew and adding a little
configuration to my .tmux.conf file fixes this problem for me.

More info: https://github.com/atom/atom/issues/3093