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)

.travis.yml

@@ -1,17 +1,15 @@
 sudo: false
 language: python
 dist: xenial
+services:
+  - mysql
+  - postgresql
 
 matrix:
   include:
     - python: 2.7
       env: TOXENV=py27-django111
 
-    - python: 3.4
-      env: TOXENV=py34-django111
-    - python: 3.4
-      env: TOXENV=py34-django20
-
     - python: 3.5
       env: TOXENV=py35-django111
     - python: 3.5