Add travis-ci for sugardough reference project.

FYI, I've used the `shields.io`versions of the SVG in https://raw.githubusercontent.com/mozilla/sugardough/master/sugardough/README.md:

```
[![Build Status](https://img.shields.io/travis/mozilla/sugardough/master.svg)](https://travis-ci.org/mozilla/sugardough)
[![Coverage status](https://img.shields.io/coveralls/mozilla/sugardough/master.svg)](https://coveralls.io/r/mozilla/sugardough)
```