Feature/upgrade to rails 4.2

due to refactor in rails/rails@f8d2899d12d59360f29c5eb6a1b1a8fe4ec82ca0 `build_association` was moved out of `ActiveRecord::Reflection::AssociationReflection` and into `ActiveRecord::Reflection::AbstractReflection`. This would cause a `wrong argument error (2 for 1)` when building or creating objects in a has many through association, i.e `@group.members.build`.

I've added a test which would cause the break, and the change which passes all tests.

I believe this should be a minor level version bump as the change will cause a break in Rails < 4.2.

 Thanks to @JGRUBBS for helping me in the process of narrowing down this bug.