Ability to use tmate with tmuxinator is broken in recent release

As mentioned in post closure comments on #327 this was working for a while but has been broken in a more recent release. As a work around users may use the `debug` feature to show the commands, save the results to a file, and remove `TMUX=` from one of the lines to get it to work again but long term it would be great for this feature to work again without that workaround.