Use python2 and python3

Author: chdemko

.travis.yml

@@ -13,12 +13,8 @@ install:
   - pip install tox-travis
 script:
   - tox
-  - pip install pip2
-  - pip install pip3
-  - pip2 install coverage
-  - pip3 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
+  - python2 `which coverage` run -a --source=pandoc_codeblock_include setup.py test
+  - python2 `which coverage` run -a --source=pandoc_codeblock_include setup.py test
   - coverage report -m
 after_success:
   - coveralls