add pytest-cov dependencie

.travis.yml

@@ -7,6 +7,7 @@ python:
   - "3.6"
 install:
   - "pip install -r requirements.txt"
+  - "pip install pytest-cov"
   - "pip install coveralls"
 script:
   - py.test --cov-config .coveragerc --cov=synergine2 ./tests/