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)
David Wobrock
Felix