Add Shoulda Matchers 3 configuration

From version 3.0.0 it is needed to specfied which test framework you're
using and which portion of the matchers you want to use in
`rails_helper.rb` (or the old `spec_helper.rb` in our case).

spec/spec_helper.rb

@@ -122,3 +122,10 @@ RSpec.configure do |config|
 end
 
 OmniAuth.config.test_mode = true
+
+Shoulda::Matchers.configure do |config|
+  config.integrate do |with|
+    with.test_framework :rspec
+    with.library :rails
+  end
+end