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!
Ah yes, this started life in the `ShoesCommonManifest` class where it would have been necessary before I realized it wasn't getting subclassed by the others. Will try that out, but I'm pretty sure you're right about it.