Fix .travis.yml to add grunt-cli because travis needs grunt to run $ grund pre-build for e2e test

.travis.yml

@@ -3,8 +3,9 @@ node_js:
   - 'stable'
   - 'lts/*'
 before_script:
+  - 'npm install grunt-cli -g'
   - 'export DISPLAY=:99.0'
   - 'sh -e /etc/init.d/xvfb start'
-  - sleep 3
+  - 'sleep 3'
 
 script: npm run lint && npm run test && grunt pre-build && npm run test:e2e