Include certain helpers only for controller specs

Travis CI was failing because of the conflict between the `sign_out` method
meant for integration tests and the `sign_out` method used for controller specs.

Example failure: https://travis-ci.org/thoughtbot/clearance/jobs/4730477

lib/clearance/testing.rb

@@ -17,6 +17,6 @@ class ActionController::TestCase
 if defined?(RSpec) && RSpec.respond_to?(:configure)
   RSpec.configure do |config|
     config.include Clearance::Testing::Matchers
-    config.include Clearance::Testing::Helpers
+    config.include Clearance::Testing::Helpers, :type => :controller
   end
 end