Apply fixes to Travis CI configuration.

.travis.yml

@@ -5,10 +5,10 @@ env:
 addons:
   firefox: latest
 before_install:
-  - sudo apt-get install -y python3-pillow
+  - sudo apt-get install -y python3-pillow python3-numpy
 install:
   - wget https://github.com/mozilla/geckodriver/releases/download/v0.24.0/geckodriver-v0.24.0-linux64.tar.gz
-  - tar -xf geckodriver-v0.20.0-linux64.tar.gz
+  - tar -xf geckodriver-v0.24.0-linux64.tar.gz
   - sudo mv geckodriver /usr/local/bin
 jobs:
   include: