Update Travis.yml to use supported node versions

[mbernier]

Issue Summary

The nodeJS releases site shows that .10 and .12 are no longer supported. They also show some major builds that are supported. Please update Travis.yml accordingly.

Example: sendgrid/smtpapi-nodejs#37

[mbernier]

@rasjani would you be up for making a PR against our repo for that change? We'd love to give you credit!!

[rasjani]

I actually did - before i noticed #22 which was essentially the same patch but from different author =)