allow_mass_assignment is no longer auto-required

Since allow_mass_assignment tests code that has moved out of Rails and
into the protected_attributes gem, there is no reason to include this by
default.

You can use this into your tests now by saying:

   require 'shoulda/matchers/protected_attributes'

   RSpec.configure do |config|
     config.include(Shoulda::Matchers::ProtectedAttributes)
   end