* Misspelled filename => fail (instead of linting all migrations and ignoring the error)
* An empty file/a file with no valid migration paths => lint no migrations (instead oflinting all migrations)
* If the option is not specified, keep the behaviour of linting all migrations
Instead of using the file path when the git commit is not
added, use the file and the migrations list no matter if we select
all migrations or just from a certain git commit.
And unit tests :)
* 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)