ci: Fix coveralls report

`cpp-coverall` overwrites the binary `coveralls` which doesn't work
exactly as the original, but which we would like to have for reporting.

Uses the module approach to really use the "original" coveralls.

.travis.yml

@@ -27,7 +27,7 @@ script:
 after_success:
   - coverage report
   - codecov
-  - coveralls --merge=cpp-coveralls.json
+  - python -m coveralls --merge=cpp-coveralls.json
   - ocular --data-file ".coverage"
 before_deploy:
   # Builds wheels only with Python 3.7