Make Travis-CI use Tox for test matrix execution

see http://jsatt.com/blog/using-tox-with-travis-ci-to-test-django-apps/

.travis.yml

@@ -1,26 +1,22 @@
 language: python
-python:
-  - "2.6"
-  - "2.7"
-  - "3.3"
-  - "3.4"
-env:
-  - DJANGO="Django>=1.4,<1.5"
-  - DJANGO="Django>=1.5,<1.6"
-  - DJANGO="Django>=1.6,<1.7"
-  - DJANGO="Django>=1.7,<1.8"
-  - DJANGO="Django>=1.8,<1.9"
-matrix:
-  exclude:
-    - python: "2.6"
-      env: DJANGO="Django>=1.7,<1.8"
-    - python: "2.6"
-      env: DJANGO="Django>=1.8,<1.9"
-    - python: "3.3"
-      env: DJANGO="Django>=1.4,<1.5"
-    - python: "3.4"
-      env: DJANGO="Django>=1.4,<1.5"
 install:
-  - pip install $DJANGO
+  - pip install tox
 script:
-  - python setup.py test
+  - tox
+env:  # generate list with: $ tox -l | xargs -I ITEM echo "  - TOXENV="ITEM
+  - TOXENV=py26-django14
+  - TOXENV=py26-django15
+  - TOXENV=py26-django16
+  - TOXENV=py27-django14
+  - TOXENV=py27-django15
+  - TOXENV=py27-django16
+  - TOXENV=py27-django17
+  - TOXENV=py27-django18
+  - TOXENV=py33-django15
+  - TOXENV=py33-django16
+  - TOXENV=py33-django17
+  - TOXENV=py33-django18
+  - TOXENV=py34-django15
+  - TOXENV=py34-django16
+  - TOXENV=py34-django17
+  - TOXENV=py34-django18