adding travis CI

Adding travis CI hook. We would be able to see build healths from the repo page.
The script seems to work. The tests don't finish execution because of the zookeeper url 404ing (submitted fix in other PR : https://github.com/ynachiket/node-zk-ultralight/commit/cb02fc776403b1851bedd39eef66779a19875da7)

https://travis-ci.org/ynachiket/node-zk-ultralight/jobs/16341729