Allow later minor revisions of Thor gem

Most of our repos will already be on 0.19.1 because it's required by Rails
3.2.18.  Thor's changelog hasn't been updated since 0.18.1, but in theoretically
0.19.1 should be compatible.
Merge pull request #1 from Yesware/allow-thor-minor-version-updates

Allow later minor revisions of Thor gem