also install geckodriver on travis

Author: Gregg Van Hove

.travis.yml

@@ -1,6 +1,11 @@
 language: ruby
 cache: bundler
 sudo: false
+before_install:
+  - wget https://github.com/mozilla/geckodriver/releases/download/v0.19.1/geckodriver-v0.19.1-linux64.tar.gz
+  - mkdir geckodriver
+  - tar -xzf geckodriver-v0.19.1-linux64.tar.gz -C geckodriver
+  - export PATH=$PATH:$PWD/geckodriver
 before_script:
 - sh -e /etc/init.d/xvfb start
 script: DISPLAY=:99.0 bundle exec rake