Run tests on Python 3.6

Unfortunately, both Python 3.5 and Python 3.6 are not available on
Travis CI by default. That's why we can't just run `tox` and be happy.
Instead, we need to use Travis feature to tell explicitly which
environment we want to consume by means of using matrix.

.travis.yml

@@ -1,6 +1,19 @@
 sudo: false
 language: python
-python: 3.5
+
+matrix:
+  include:
+    - python: 2.7
+      env: TOXENV=py27
+    - python: 3.3
+      env: TOXENV=py33
+    - python: 3.4
+      env: TOXENV=py34
+    - python: 3.5
+      env: TOXENV=py35
+    - python: 3.6
+      env: TOXENV=py36
+    - env: TOXENV=docs
 
 install:
    - pip install tox

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27, py33, py34, py35, docs
+envlist = py27, py33, py34, py35, py36, docs
 
 [testenv]
 deps =