Add workaround for coveralls on Python 3.6

Apparently, for some reason yet to be determined, it appears that
`coveralls` cannot upload the results from `coverage` on Python 3.6. It
looks like `coverage` may not be able to read what it wrote :/. So here
we try to install Python 3.5 before uploading `coverage`'s results.

.travis.yml

@@ -38,6 +38,7 @@ script:
 
 # Report coverage
 after_success:
+  - if [[ "${PYVER}" == "3.6" ]]; then conda install -yq python=3.5 --file environment_ci.yml; fi
   - coveralls
 
 # Disable email notifications.