travis: run tests both with and without cython

.travis.yml

@@ -1,14 +1,23 @@
 sudo: false
 language: python
-python:
-  - "2.7"
-  - "3.6"
+
+matrix:
+  include:
+  - env: TOXENV=py27-nocy
+    python: 2.7
+  - env: TOXENV=py36-nocy
+    python: 3.6
+  - env: TOXENV=py27-cy
+    python: 2.7
+  - env: TOXENV=py36-cy
+    python: 3.6
+
 branches:
   only:
     - master
     - /^v\d+\.\d+.*$/
 
-install: pip install tox-travis
+install: pip install tox
 
 script: tox