Add Travis CI support

[lirantal]

Description

Update the project with a proper travis ci file to run the tests (this is a good place to also check several Node.js versions so we can track when it breaks)

Requirements

The project already has integration with Travis CI but it's missing the proper travis file with the right definition:

1. Node.js project
2. Test the following versions: 4, 6, and 8.

[civan]

let me take a look at this one