Tests that running the linter a second time by ignoring an erroneous migration
will not hit the cache for the said migration.
Instead, it is correctly ignored.
* don't use the project fixtures anymore but the one Django project
* update and refactor cache tests to be less brittle
* explicitly test mysql, postgresql and sqlite3 databases
* remove 'multiple_call' tests since it is not possible anymore
(with the Django singleton)
* update Travis and tox (dropping Python 3.4)