Another Travis CI configuration fix.

.travis.yml

@@ -6,6 +6,7 @@ addons:
   firefox: latest
 before_install:
   - sudo apt-get install -y python3-pillow python3-numpy
+  - sudo pip3 install selenium
 install:
   - wget https://github.com/mozilla/geckodriver/releases/download/v0.24.0/geckodriver-v0.24.0-linux64.tar.gz
   - tar -xf geckodriver-v0.24.0-linux64.tar.gz