use shared_examples_for versioning, for both posts and topics

spec/support/versioning/shared_examples_for_versioned_model.rb

@@ -0,0 +1,12 @@
+shared_examples_for "a versioned model" do
+  let(:model) { Fabricate(described_class.to_s.downcase) }
+  let(:first_version_at) { model.last_version_at }
+
+  it 'should be versioned' do
+    model.should respond_to(:version)
+  end
+
+  it 'has an initial version of 1' do
+    model.version.should == 1
+  end
+end