chore: use headless firefox for Travis (#156)

Author: linuxwolf

.travis.yml

@@ -18,13 +18,9 @@ matrix:
   include:
   - node_js: 8
     env:
-    - CXX=g++-4.8 TASKS=travis:browser
+    - CXX=g++-4.8 TASKS=travis:browser MOZ_HEADLESS=1
     addons:
       firefox: latest
       sauce_connect: true
-before_script:
-- "export DISPLAY=:99.0"
-- "sh -e /etc/init.d/xvfb start"
-- sleep 5 # give xvfb some time to start
 script:
 - ./node_modules/.bin/gulp $TASKS