update travis.yml to run grunt build task

[vukasin-nikodijevic]

Currently travis config requires only node v0.10 but I don't see that it runs grunt build task.

To have repo build 100% VALID, travis MUST run grunt task right after npm install.

[addyosmani]

@Vukasin0 Thanks for noticing that. Is the suggestion here to add:

after_success:
  - grunt build

to the travis config?

[vukasin-nikodijevic]

Here is good example:
https://stackoverflow.com/questions/21128478/run-grunt-build-command-on-travis-ci