Travis: avoid duplicate builds

Change .travis.yml to generate a build matrix which is only
TRAVIS_PYTHON_VERSION x DJANGO_VERSION, reducing the number of builds
from 15 to 5.

.travis.yml

@@ -6,20 +6,20 @@ python:
   - "3.3"
 
 env:
-  - TOXENV=py26-1.5
-  - TOXENV=py27-1.5
-  - TOXENV=py33-1.5
-  - TOXENV=docs
-  - TOXENV=py27-dev,py33-dev
+  - DJANGO_VERSION=1.5
+  - DJANGO_VERSION=dev
 
 matrix:
   allow_failures:
-    - env: TOXENV=py27-dev,py33-dev
+    - env: DJANGO_VERSION=dev
+  exclude:
+    - env: DJANGO_VERSION=dev
+      python: "2.6"
 
 # command to install dependencies
 install:
     - pip install tox
 
 # command to run tests
 script:
-    - tox
+    - tox -e py${TRAVIS_PYTHON_VERSION/./}-${DJANGO_VERSION},docs