Fix join association on Active Record 4.1.x

Update the overridden build_constraint method to match changes made in
Active Record 4.1.0.
Merge pull request #189 from simonsouth/bug/fix-join-association-on-ar-4.1.x

Fix join association on Active Record 4.1.x
Merge remote-tracking branch 'upstream/ar_4.1.x' into ar41

* upstream/ar_4.1.x:
  Fix join association on Active Record 4.1.x