Rename buffet-gem to buffet

The authors of the `buffet` gem were kind enough to relinquish the
namespace for us to use.

Rename the gem to `buffet` so we can push it to that namespace. We don't
need to update the version since the `buffet` namespace doesn't have any
versions posted.

Change-Id: I9539b3c50ed7c95947f03e9d0e177848e38edc60
Reviewed-on: https://gerrit.causes.com/30888
Reviewed-by: Adam Olsen <adam.olsen@causes.com>
Reviewed-by: Joe Lencioni <joe.lencioni@causes.com>
Tested-by: Shane da Silva <shane@causes.com>