Only test main branch, reduce number of builds on Travis CI.
Rename QuestionPlugin.entityProperties to .properties
Issue #2383141 by thehong: Rename scale module to quizz_scale and clean it up:

- Rename scale -> quizz_scale
- {quiz_scale_node_properties} -> {quiz_scale_properties}
- {quiz_scale_properties}.nid => {quiz_scale_properties}.qid
- {quiz_scale_user_answers}.question_nid -> {quiz_scale_user_answers}.question_qid
- Add more columns for {quiz_scale_collections}: name, label, uid, data, module, status
- Provide entity api for scale collection
- Move logics to collection's controller
- Drop {quiz_scale_user} table.
- Convert global configuration for questions to question type entity.
- Update test cases
- Make use of new entity API in quizz_scale_install()