Adding django-filter to .travis.yml

As pip doesn't seem inclined to pickup on tests_require

Author: adamcharnock

.travis.yml

@@ -23,7 +23,7 @@ before_install:
 install:
 - |
   if [ "$TEST_TYPE" = build ]; then
-    pip install --download-cache $HOME/.cache/pip/ pytest pytest-cov coveralls six pytest-django
+    pip install --download-cache $HOME/.cache/pip/ pytest pytest-cov coveralls six pytest-django django-filter
     pip install --download-cache $HOME/.cache/pip/ -e .[django]
     python setup.py develop
   elif [ "$TEST_TYPE" = build_website ]; then