chore(tests): run against Firefox, too

.travis.yml

@@ -1,9 +1,17 @@
 language: node_js
 node_js: '4.0'
 
+addons:
+  firefox: "latest-esr"
+
+before_script:
+  - "export DISPLAY=:99.0"
+  - "sh -e /etc/init.d/xvfb start"
+  - sleep 3
+
 script:
-  - npm run all
+  - TEST_BROWSERS=Firefox,PhantomJS npm run all
 
 before_install:
-  - npm install -g wiredeps
-  - wiredeps --branch=$TRAVIS_BRANCH --tag=$TRAVIS_TAG --verbose
+  - npm install -g grunt-cli wiredeps
+  - wiredeps --branch=$TRAVIS_BRANCH --tag=$TRAVIS_TAG --verbose