Optimise travis config

.travis.yml

@@ -8,10 +8,7 @@ addons:
     chrome: stable
 node_js:
  - 6
-
-before_install:
-    - export DISPLAY=:99.0
-install: make dev
+install: make stamp-npm
 before_script: make serve_bg
 script: make check
 sudo: false