Add and apply RSpec/BeEq rubocop rule

This rule was added in rubocop-rspec 2.9.0. Once again, we were applying
it about 50% of the time.

spec/models/poll/answer_spec.rb

@@ -76,7 +76,7 @@ describe Poll::Answer do
 
       expect(voter.document_number).to eq(answer.author.document_number)
       expect(voter.poll_id).to eq(answer.poll.id)
-      expect(voter.officer_id).to eq(nil)
+      expect(voter.officer_id).to be nil
     end
 
     it "updates a poll_voter with user and poll data" do