add py36 to build matrix

.travis.yml

 sudo: false
 language: python
-python: 2.7
 
 # tell travis to cache ~/.cache/pip
 cache: pip
 
-env:
-  - TOX_ENV=packaging
-  - TOX_ENV=pep8
-  - TOX_ENV=py27
+matrix:
+  include:
+  - python: 2.7
+    env: TOX_ENV=packaging
+
+  - python: 2.7
+    env: TOX_ENV=pep8
+
+  - python: 2.7
+    env: TOX_ENV=py27
+    
+  - python: 3.6
+    env: TOX_ENV=py36
 
 install:
   - pip install tox