Rename trait with_answers to yes_no

We barely use this trait. In the votation type spec we're probably using
it wrong, and in the answer spec we assume one of the answers is going
to be "Yes".

The name `yes_no` is more expressive, since it makes it clear what the
answers are.

spec/models/poll/answer_spec.rb

@@ -43,7 +43,7 @@ describe Poll::Answer do
 
     let(:author) { create(:user, :level_two) }
     let(:poll) { create(:poll) }
-    let(:question) { create(:poll_question, :with_answers, poll: poll) }
+    let(:question) { create(:poll_question, :yes_no, poll: poll) }
 
     it "creates a poll_voter with user and poll data" do
       answer = create(:poll_answer, question: question, author: author, answer: "Yes")