Remove binstubs so they aren't packaged in .gem

Because we list the files to package in this Gem by shelling out to Git,
including all files that are in Git, we want to exclude our binstubs. If
we don't those binstubs get packaged with our gem, and so if you install
the gem, our binstubs end up on your PATH. OOPS!

An alternative would be to change how we list the "files" in our
.gemspec to make sure our binstubs are excluded. Maybe later...