store_base_sti_class_for_4_1

Once Activerecord 4.1 has a bunch of changes related to refactoring, the previous version of the gem is not compatible with it. The commits contain updated version of the gem due to the latest changes of AR, mostly placed in store_based_class_for_sti_4_1 according to the general conception. Algorithm of indication which version should be applied was slightly modified as well for versions of AR >= 4. 
Tests passed successfully.