Add Travis CI configuration

[md5]

This PR adds configuration for Travis CI.

The tests on my Travis setup are currently failing, but that's because getEndpoint is calling exists on /var/run/docker.sock and Docker isn't running in the Travis environment. I have a similar issue with that same test failing locally because I'm running boot2docker and my DOCKER_HOST is set to point to my b2d VM.

Travis would still need to be turned on for jwilder/docker-gen for this to work.

[datacoda]

I think you might be able to move the (3?) getEndpoint related tests into their own file with // +build !travis as a guard?

[md5]

I'd actually prefer getEndpoint to not call exists. It doesn't do any
similar checks on a tcp:// DOCKER_HOST.

[jwilder]

Cool. I've turned it on for jwilder/docker-gen. We can probably move the exist check out of the getEndpoint. It might not even be necessary since trying to open it will still fail and should already be handled.

[md5]

Yeah, I think if the exists check isn't there, then a connection to a missing Unix socket will fail at the same time as a bad TCP connection. That seems appropriate to me.

[md5]

I also just realized that I might have added the build status badge to README.md. The markdown looks like this

[![Build Status](https://travis-ci.org/jwilder/docker-gen.svg?branch=master)](https://travis-ci.org/jwilder/docker-gen)

[jwilder]

Added