Specify Python versions to use in travis.

This should address python2.6 being missing in the default environment. Fixes #332

.travis.yml

@@ -1,15 +1,20 @@
 language: python
-python: 2.7
-env:
-  - TOX_ENV=py26
-  - TOX_ENV=py27
-  - TOX_ENV=py33
-  - TOX_ENV=py34
-  - TOX_ENV=squeeze
-  - TOX_ENV=wheezy
 matrix:
   allow_failures:
     - env: TOX_ENV=squeeze
+  include:
+    - python: 2.6
+      env: TOX_ENV=py26
+    - python: 2.7
+      env: TOX_ENV=py27
+    - python: 3.3
+      env: TOX_ENV=py33
+    - python: 3.4
+      env: TOX_ENV=py34
+    - python: 2.6
+      env: TOX_ENV=squeeze
+    - python: 2.7
+      env: TOX_ENV=wheezy
 install:
   - pip install -U pip
   - pip install tox