Also run doctests in Travis-CI runs to match ./run_tests.sh

Author: Stephan Sokolow

.travis.yml

@@ -26,6 +26,7 @@ env:
 script:
   - mypy quicktile ./*.py functional_harness
   - nosetests3
+  - python3 -m doctest quicktile/util.py
 after_success: coveralls
 deploy:
   provider: script