Working to bring up to Celery 3.1 API happiness

- Moved phpunit.ini => phpunit.ini.dist
- Fixed PHPAmqp Connector to make virtual host config behaviour uniform
  with celery as `zircote/amqp`
- Dropping support for MongoDB backend keeping inline with Celery
  roadmap see: http://docs.celeryproject.org/en/latest/whatsnew-3.1.html#preface
- updated `.travis.yml` for test execution to drop mongo support
- updated composer.json: removed "suggests" ext-mongo