command/rsync-auto: properly exit on interrupt

Currently, interrupting 'vagrant rsync-auto' once it's waiting for
filesystem events has no effect. Appears to be a deadlock related to
signal handlers in Ruby 2.0 [0].

Remove call to `thread.join` in signal handler. The handler doesn't
need to wait for `true` to be added to the queue; it just needs to
launch the thread.

[0]: https://bugs.ruby-lang.org/issues/7917
Merge pull request #3552 from benesch/rsync-auto-handle-interrupt

command/rsync-auto: properly exit on interrupt