Enable RSpec/NotToNot cop and fix all issues

Read about cop at http://www.rubydoc.info/gems/rubocop-rspec/RuboCop/Cop/RSpec/NotToNot

spec/lib/census_api_spec.rb

@@ -60,7 +60,7 @@ describe CensusApi do
       allow(api).to receive(:get_response_body).with(1, "0123456").and_return(invalid_body)
       response = api.call(1, "123456")
 
-      expect(response).to_not be_valid
+      expect(response).not_to be_valid
     end
   end