Run tests on MySQL and use travis-$DB environments.

This PR makes Travis use the travis-sqlite3 and travis-mysql environments.

- I had to adjust ports in config (see #1391)
- `core/server.js` now shows the environment Ghost is running in
- I had to change testUtils since MySQL foreign key constraints failed on posts_tags (apparently when.all runs in parallel and not all posts/tags where inserted when posts_tags was executed)

refs #921
fixes #1385