Add Travis CI

[BaptisteDixneuf]

Integrate your GitHub project with Travis-ci

• First go to https://travis-ci.org/ and sign in with your GitHub account.
• In the upper right corner click on your name (or choose Accounts) to open your Travis-ci profile.
  You'll be presented with the list of your GitHub projects (only the ones where you have administrative authority)
• Toggle the switch on one of your projects and then click on little wrench icon to open the project's Service Hooks page on GitHub.
  To get to the same place from your GitHub project, go to Settings > Service Hooks.
• On the Service Hooks page scroll down to Travis and you'll notice that the hook is already activated. Click on "Travis".
• Click on Test Hook.

[elisee]

OK wow this is going to be very cool! Thanks ^_^. We'll take care of it soon!

[elisee]

Merged manually as 7a565b5 to fix the path of the badge.

@BaptisteDixneuf do you know if we can get it to install npm@3? or would that require using node@5?

EDIT: nevermind, found how to enforce it! af3bef6