Enable Travis CI integration.

See https://travis-ci.org/

The changes here enable Travis CI integration for https://github.com/jimlloyd/gremlin-node.
Anthony will need to setup his Travis account and then edit the line in README.md to
reference his own repo.

I.e. replace jimlloyd with inolen in this bit of markdown:
[![Build Status](https://travis-ci.org/jimlloyd/gremlin-node.svg)](https://travis-ci.org/jimlloyd/gremlin-node)