Remove reattach-to-user-namespace

This is Mac OS X-specific. Including this line in the dotfiles causes
`tmux` and `tmux new -s new-session` to break with `[exited]`.

I personally don't do copy-pasteable work inside tmux anymore.

I'm sure this is valuable to some of the thoughtbot vim users but I'm
thinking it better belongs above `DO NOT EDIT BELOW THIS LINE` as custom
configuration for those users.