Specify language as Node.js on Travis CI

Without `language` section, Travis CI detects Ruby as build language:
* https://travis-ci.org/emberjs/data/builds/24150621#L18
Use npm-scripts to run test

* https://www.npmjs.org/doc/misc/npm-scripts.html
Merge pull request #1906 from tricknotes/install-grunt-on-travis

Fix failing test on Travis CI.