Fix simplecov and coveralls support.

Since Vagrant (and thus this plugin) run under a separate process,
we need to trigger simplecov from within our plugin. We then have
to have coveralls retrieve and send the results. We accomplish this
by setting and environmental variable at the beginning of our tests
which then triggers simplecov at the top of our plugin. Coveralls is
then set up to merge the results (although there is only one set)
and submit the results under its own rake task.