CI: Attempt to builds docs during regular CI, too

Since ReadTheDocs does not support [1] running as a CI check on GitHub
yet, let's make sure that we at least run sphinx. This would have caught
a recent docs breakage (see parent commit).

[1] https://github.com/readthedocs/readthedocs.org/issues/1340

.travis.yml

@@ -13,5 +13,6 @@ script:
   - rstcheck --ignore-roles cite --ignore-directives automodule --recursive --ignore-messages '(Duplicate explicit target name.*)' .
   - ./examples/transmission_main_example.py
   - ./examples/path_requests_run.py
+  - sphinx-build docs/ x-throwaway-location
 after_success:
   - bash <(curl -s https://codecov.io/bash)