Fix compatibility with Rails 4/Arel 4.0.1.

The method signature of the Arel stuff changed in the 4.0.1 release:
https://github.com/rails/arel/commit/68a95542e1a7a79d9777223fbffd2b982fed0268

This adds some basic tests surrounding this Arel functionality using a
FakeRecord class taken from how Arel performs its own tests.
Merge pull request #12 from GUI/arel401

Fix compatibility with Rails 4/Arel 4.0.1