Fix to uninstall shoes and test our manifests

We generate two non-Ruby files in the Gem bin dir--shoes (during gem
install) and shoes-backend (during the first run or picking a shoes
backend).

The shoes binary wasn't getting uninstalled because we weren't putting the
lib/rubygems_plugin.rb file into the gem itself. That plugin is what's
responsible at gem uninstallation time for removing shoes since rubygems
itself doesn't know about the shoes executable.

shoes-backend was a recently added file that got missed on adding to
cleanup when I created it.

We've had at least two problems resulting from files in the repo that
didn't get added into a manifest and hence were missing from the gem.
Now there's a test that will confirm that every .rb file in the repo
shows up in at least one of the manifests.

Time to start testing for it!