Update 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)

tox.ini

@@ -1,12 +1,11 @@
 [tox]
 envlist =
-    py{27,34,35,36,37}-django111
-    py{34,35,36,37}-django20
-    py{35,36,37}-django21
+    py27-django111
+    py{35,36,37}-django{111,20,21}
     lint
 
 [testenv]
-commands = py.test {posargs}
+commands = ./manage.py test --no-input {posargs}
 extras = test
 pip_pre = true
 deps =
@@ -23,4 +22,4 @@ deps =
     black
 commands =
     flake8 --max-line-length=88 django_migration_linter
-    black --check django_migration_linter
+    black --check django_migration_linter/ tests/ manage.py setup.py