Add verbosity to tox command to pass on as pip -vvv

.travis.yml

@@ -17,7 +17,9 @@ matrix:
         - sudo apt-get update
         - sudo apt-get install -y build-essential python3-dev libgtk-3-dev libjpeg-dev libtiff-dev
       script:
-        - tox
+        # Pass -vvv to pip through tox because wxPython can take ~20min to build and
+        # Travis CI thinks that the build has stalled unless build progress is output.
+        - tox -vvvvv
     - python: 3.7
       name: "Test Python 3.7"
       env: TOXENV=py37