Use built-in services in CI

[arm4b]

Low priority

CircleCI has built-in RabbitMQ, Mongo and PostgreSQL services already up and running.
Instead of downloading respective Docker images, we can use CI services to save some time (2-3 mins).

Since docker-compose supports different configs, depending on environment, this looks like good approach which won't break existing beauty.
http://docs.docker.com/compose/extends/#multiple-compose-files

So for local env and CI env there will be a bit different configs (with and without linked services).

[dennybaa]

Yeap. Btw we already using multiple compose files.

[dennybaa]

implemented in #17

[arm4b]

👍