`dep` aggressively prunes everything under `vendor/`. Unfortunately,
this consistently nukes the contents of `vendor/bundle` directory.

These were both requested during code review.

This is helpful for when TMPDIR already has a trailing slash. We can't
be sure that it will on all operating systems, but this will remove it
if it is there.

Since we're no longer connected to a tty, we can't start up tmux.