Use coverage for python2 and python3

Author: chdemko

.travis.yml

@@ -13,6 +13,8 @@ install:
   - pip install tox-travis
 script:
   - tox
+  - python2 -m pip install coverage
+  - python3 -m pip install coverage
   - python2 -m coverage run -a --source=pandoc_codeblock_include setup.py test
   - python3 -m coverage run -a --source=pandoc_codeblock_include setup.py test
   - coverage report -m