Migrate from legacy to container-based Travis infrastructure

See https://docs.travis-ci.com/user/migrating-from-legacy/?utm_source=legacy-notice&utm_medium=banner&utm_campaign=legacy-upgrade

.travis.yml

@@ -1,8 +1,9 @@
 language: node_js
+sudo: false
 node_js:
   - "0.12"
 before_install: npm install -g grunt-cli
 install: npm install
 before_script: grunt
 script: 
- - grunt coveralls
+ - grunt coveralls