Change to run Travis CI tests on Python 3.6

This will make it so that we test under the same Python version as AWS
Lambda uses (at least if the user's chosen to use 3.6 rather than 2.7).

.travis.yml

@@ -1,6 +1,6 @@
 language: python
 python:
-  - "3.5"
+  - "3.6"
 install:
   - "pip install flake8"
   - "pip install requests"