Fix codecov's coverage not reported in CI build status

Not sure what's happening here. It seems the Github status icon
displaying build status is not updated by codecov, except if some time
has passed between ``codecov`` command and end of Travis build
(previously, this was working thanks to ``cd docs && make html``).

.travis.yml

@@ -17,3 +17,4 @@ script: tox
 
 after_success:
   - codecov --flags "py_${TRAVIS_PYTHON_VERSION//[^[:alnum:]]/}"
+  - sleep 10  # Github build status would not be updated by "codecov" otherwise