Disable allow_mass_assignment_of tests under Rails 5

The `allow_mass_assignment_of` matcher tests `attr_accessible` and
`attr_protected` which was moved to the `protected_attributes` gem in
Rails 4. The gem does not work in Rails 5 and neither does the matcher,
so there is no need to test it. (We should probably remove the matcher
altogether eventually -- Strong Parameters have long replaced
`protected_attributes`.)

spec/support/unit/helpers/rails_versions.rb

@@ -30,5 +30,9 @@ module UnitTests
     def rails_gte_4_2?
       rails_version >= 4.2
     end
+
+    def rails_lte_5?
+      rails_version < 5
+    end
   end
 end

spec/unit/shoulda/matchers/active_model/allow_mass_assignment_of_matcher_spec.rb

@@ -1,6 +1,7 @@
 require 'unit_spec_helper'
 
 describe Shoulda::Matchers::ActiveModel::AllowMassAssignmentOfMatcher, type: :model do
+  if rails_lte_5?
     context '#description' do
       context 'without a role' do
         it 'includes the attribute name' do
@@ -112,4 +113,5 @@ describe Shoulda::Matchers::ActiveModel::AllowMassAssignmentOfMatcher, type: :mo
     def define_model(name, columns, &block)
       super(name, columns, whitelist_attributes: false, &block)
     end
+  end
 end