add CI test for python3.5

Python3.5 is still the default Python3 version in
Debian Stretch (stable) and Ubuntu 16.04 LTS
https://packages.debian.org/source/stretch/python3-defaults
https://packages.ubuntu.com/source/xenial/python/python3-defaults

.travis.yml

@@ -9,6 +9,7 @@ addons:
 language: python
 python:
   - "2.7"
+  - "3.5"
   - "3.6"
 before_install:
   - "export DISPLAY=:99.0"