add python-control tests to coverage

Author: bnavigator

.travis.yml

@@ -146,7 +146,7 @@ script:
   # Get python-control master branch
   - git clone --depth 1 https://github.com/python-control/python-control.git control
   - cd control
-  - pytest --disable-warnings
+  - pytest --disable-warnings --cov=$slycot_dir --cov-config=../Slycot/.coveragerc
 
 after_success:
   - coveralls