Try to fix travis-ci missing python versions with tox-travis.

.travis.yml

@@ -9,21 +9,25 @@ addons:
     - libgdal-dev
 
 language: python
-python: 3.6
-env:
-  - TOX_ENV=quality-check
-  - TOX_ENV=pypy
-  - TOX_ENV=py36
-  - TOX_ENV=py35
-  - TOX_ENV=py34
-  - TOX_ENV=py33
-  - TOX_ENV=py27
-  - TOX_ENV=docs
+python:
+  - "2.7"
+  - "3.3"
+  - "3.4"
+  - "3.5"
+  - "3.6"
+  - "pypy"
+matrix:
+  fast_finish: true
+  include:
+  - python: 3.5
+    env: TOXENV=docs
+  - python: 3.5
+    env: TOXENV=quality-check
 
 install:
-  - pip install tox
+  - pip install tox tox-travis
   - pip install coveralls
-script: tox -e $TOX_ENV
+script: tox
 after_success:
   - if [[ $TOX_ENV == "quality-check" ]]; then
       coveralls;