Install firefox using the travis ci firefox addon instead of messing with apt ourselves

Author: Joris-van-der-Wel

.travis.yml

@@ -3,7 +3,7 @@ node_js:
     - "node"
 env:
     global:
-        - TEST_FIREFOX_BIN=/usr/bin/firefox
+        - TEST_FIREFOX_BIN="${TRAVIS_HOME}/firefox-latest-unsigned/firefox/firefox"
         - TEST_HEADLESS=1
 dist: xenial
 install:
@@ -19,16 +19,8 @@ jobs:
 
         - stage: test
           env: TEST_SUITE=unit,integration
-          # the firefox deb download from the ppa is very very slow (~3 minutes), cache it
-          cache:
-              directories:
-                  - /home/travis/apt-firefox-cache
-          before_install:
-              - sudo add-apt-repository -y ppa:ubuntu-mozilla-daily/firefox-aurora
-              - sudo chown --recursive root /home/travis/apt-firefox-cache
-              - sudo apt-get update
-              - sudo apt-get install -o dir::cache::archives=/home/travis/apt-firefox-cache --no-install-recommends -y firefox
-              - sudo chown --recursive travis /home/travis/apt-firefox-cache
+          addons:
+              firefox: latest-unsigned
           script:
               - '"$TEST_FIREFOX_BIN" --version'
               - npm run test:unit:coveralls