added automated coverage output for tests

.travis.yml

@@ -1,5 +1,7 @@
- language: python
+language: python
- python:
+python:
-   - "3.6"
+  - "3.6"
- install: "pip install -r requirements.txt"
+install: "pip install -r requirements.txt"
- script: pytest --flake8 --cov=stl -x --cov-report=html
+script: pytest --flake8 --cov=stl -x --cov-report=html
+after_success:
+  - codecov