Fix unapproved unassigned MR email erroring out

Fix https://gitlab.com/gitlab-org/gitlab-ee/issues/3092

Conflicts:
	ee/app/views/notify/unapproved_merge_request_email.html.haml
	spec/mailers/notify_spec.rb

spec/mailers/previews/notify_preview.rb

@@ -65,7 +65,7 @@ class NotifyPreview < ActionMailer::Preview
   end
 
   def merge_request
-    @merge_request ||= project.merge_requests.find_by(source_branch: 'master', target_branch: 'feature')
+    @merge_request ||= project.merge_requests.first
   end
 
   def user