Added information about older versions

and about the adding of version to gemspec
removed dependency on ActiveModel

was only ever mentioned in commented out warnings
Removed unused require
Copied ActiveSupport::Concern to our codebase

This removes the dependency on ActiveSupport, and
solves a conflict between active support 3.2.14 and
Virtus, both using InstanceMethods
Merge pull request #43 from Factlink/prune-dependencies

Removed activemodel and activesupport dependencies