Enable xvfb for travis Firefox

.travis.yml

@@ -13,6 +13,10 @@ cache:
   directories:
     - node_modules
 
+before_install:
+  - "export DISPLAY=:99.0"
+  - "sh -e /etc/init.d/xvfb start"
+
 install:
   - npm install -g webdriver-manager
   - npm install -g codeclimate-test-reporter
@@ -22,11 +26,11 @@ install:
 
 before_script:
   - phantomjs -w > /dev/null &
-  - sleep 3
-  - webdriver-manager start &
-  - sleep 3
+  - sleep 2
+  - webdriver-manager start > /dev/null &
+  - sleep 2
   - npm start &
-  - sleep 3
+  - sleep 2
 
 script: npm run coverage