update coverage for Travis, add pyaml

.travis.yml

@@ -20,7 +20,7 @@ cache:
     - $HOME/.cache/pip
 
 install:
-  - pip install 'coverage<4' # coverage>=4 has issues with python3
+  - pip install coverage pyaml
   - pip install -q $DJANGO_VERSION
   - pip install -q coveralls flake8
   - pip install -r demoproject/requirements.txt