Manually define travis matrix.

Travis-CI no longer has all supported Python versions installed by default.
Each version needs to be set via the `python` setting. However, we don't
want a matrix where every Python version runs with every `TOXENV`.
Therefore, we now manually define the matrix so that the Python version
is properly matched with the TOXENV.

.travis.yml

@@ -1,12 +1,17 @@
 sudo: false
 language: python
-env:
-  - TOXENV=py27
-  - TOXENV=py33
-  - TOXENV=py34
-  - TOXENV=pypy
-  - TOXENV=flake8
-  - TOXENV=checkspelling
+matrix:
+  include:
+    - python: '2.7'
+      env: TOXENV=py27
+    - python: '3.3'
+      env: TOXENV=py33
+    - python: '3.4'
+      env: TOXENV=py34
+    - python: 'pypy'
+      env: TOXENV=pypy
+    - env: TOXENV=flake8
+    - env: TOXENV=checkspelling
 addons:
   apt:
     packages: