Try installing requirements before deploying (because the requirements that...

Try installing requirements before deploying (because the requirements that tox installed aren't available to the Python that Travis PyPI uses for deployment). Ref #934.

.travis.yml

@@ -26,6 +26,8 @@ script:
 
  - tox
 
+before_deploy:
+  - pip install -r requirements.txt
 deploy:
   provider: pypi
   # Also update server in setup.cfg