Add py36 to envlist in tox.ini

Since Lambda is compatible with Python 3.6, I thought that it would be
better to test with 3.6

Author: abetomo

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py27,py34,style
+envlist = py27,py34,py36,style
 
 [testenv]
 install_command = pip install -U {opts} {packages}