Travis CI: Add Python 3 to the testing

.travis.yml

@@ -1,6 +1,8 @@
 language: python
 python:
   - "2.7"
+  - "3.6"
 install:
-  - pip install -r requirements.txt
+  - pip install flake8 -r requirements.txt
+before_script: flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
 script: pytest