Made Xooie Rails 4 compatible

This is in preparation for a 0.1.7 patch.

Previously, Xooie required `railties`, which is not necessary and incompatible with Rails 4. This commit removes the `railties` dependency. It also bumps the version to 0.1.7, as 0.1.5 & 0.1.6 were released internally but not publicly.

Pending this PR's approval, I will tag and publish a Xooie 0.1.7 build.