Tests: Added mock library for backwards compatbility

Author: ChristianKreuzberger

.travis.yml

@@ -32,6 +32,7 @@ cache:
 install:
   - pip install -q Django==$DJANGO_VERSION # install custom django version
   - pip install -q djangorestframework==$DJANGO_REST_VERSION # install custom django rest framework version
+  - pip install -q mock==2.0.0 # install unittest mocks
 script:
   - python setup.py install
   - cd tests/

requirements_test.txt

@@ -1,2 +1,3 @@
 Django==1.10.*
-djangorestframework=3.6.*
+djangorestframework=3.6.*
+mock==2.0.0