updated version to 4.1.5 (corresponding to then current rails version)

Updated the gem to work with rails 4.
The changes resulting in this version are not backwards compatible to rails 3.
Tests are green (except one relating to different date formats of no consequence.

Overview of changes
* refactored ActiveModel  and validations
* requires ruby version -> 2+
* rails dependency -> 4.1+
* changed from test/unit to minitest
* removed dependence on attr_accessible & attr_protected consistent with rails 4