Run tests with Python3.6 on travis.

.travis.yml

@@ -1,11 +1,12 @@
 language: python
 python:
-- "2.7"
+  - "2.7"
+  - "3.6"
 install:
-- pip install -r requirements.txt
-- pip install -r test-requirements.txt
+  - pip install -r requirements.txt
+  - pip install -r test-requirements.txt
 script:
-- nosetests
-- flake8 pywebpush
+  - nosetests
+  - flake8 pywebpush
 after_success:
-- codecov
+  - codecov