Fix allow_mass_assignment specs in Rails 4

This spec has a lot of active_model version checking in it. The version
checking helpers had to be updated to account for Rails 4.

I was able to remove the check for active_model 3.0 because 3.0 is now
the lowest supported rails version.

spec/shoulda/matchers/active_model/allow_mass_assignment_of_matcher_spec.rb

@@ -59,7 +59,7 @@ describe Shoulda::Matchers::ActiveModel::AllowMassAssignmentOfMatcher do
     end
   end
 
-  unless active_model_3_2?
+  unless active_model_3_2? || active_model_4_0?
     context 'an attribute on a class with no protected attributes' do
       it 'accepts being mass-assignable' do
         no_protected_attributes.should allow_mass_assignment_of(:attr)

spec/shoulda/matchers/active_model/helpers_spec.rb

@@ -78,13 +78,11 @@ describe Shoulda::Matchers::ActiveModel::Helpers do
       end
     end
 
-    if active_model_3_0?
-      context 'if ActiveModel::Errors#generate_message behavior has changed' do
-        it 'provides the right error message for validate_presence_of' do
-          stub_active_model_message_generation(:type => :blank,
-                                               :message => 'Behavior has diverged.')
-          assert_presence_validation_has_correct_message
-        end
+    context 'if ActiveModel::Errors#generate_message behavior has changed' do
+      it 'provides the right error message for validate_presence_of' do
+        stub_active_model_message_generation(:type => :blank,
+                                              :message => 'Behavior has diverged.')
+        assert_presence_validation_has_correct_message
       end
     end
   end

spec/support/active_model_versions.rb

@@ -1,13 +1,13 @@
 RSpec.configure do |c|
-  def active_model_3_0?
-    ::ActiveModel::VERSION::MAJOR == 3 && ::ActiveModel::VERSION::MINOR >= 0
-  end
-
   def active_model_3_1?
-    ::ActiveModel::VERSION::MAJOR == 3 && ::ActiveModel::VERSION::MINOR >= 1
+    (::ActiveModel::VERSION::MAJOR == 3 && ::ActiveModel::VERSION::MINOR >= 1) || active_model_4_0?
   end
 
   def active_model_3_2?
-    ::ActiveModel::VERSION::MAJOR == 3 && ::ActiveModel::VERSION::MINOR >= 2
+    (::ActiveModel::VERSION::MAJOR == 3 && ::ActiveModel::VERSION::MINOR >= 2) || active_model_4_0?
+  end
+
+  def active_model_4_0?
+    ::ActiveModel::VERSION::MAJOR == 4
   end
 end