Add travis support

This could help, to spot a build failure.

Travis is also able to upload artifacts using github api [releases](http://docs.travis-ci.com/user/deployment/releases/). But
unfortunately travis can't cross compile by default. I see 2 ways:

- use [gox](https://github.com/mitchellh/gox) or similar
- build only for linux. registrator normally is used in a docker container, so maybe the darwin build isn't needed.