Add and apply Lint/SymbolConversion rubocop rule

This rule was added in Rubocop 1.9.0.

We're excluding the Setting model in order to keep the settings
consistent.

spec/system/admin/poll/questions/answers/images/images_spec.rb

@@ -4,7 +4,7 @@ describe "Images", :admin do
   it_behaves_like "nested imageable",
                   "poll_question_answer",
                   "new_admin_answer_image_path",
-                  { "answer_id": "id" },
+                  { answer_id: "id" },
                   nil,
                   "Save image",
                   "Image uploaded successfully",