Add Travis CI

* Use Go and [tomlv](https://github.com/BurntSushi/toml) to validate contrib sources

@johndbritton If you're interested in merging, please [configure Travis CI](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in) before we merge. Thanks!

*Local validation:*
```
~/Projects/glean-contrib travis-ci  
❯ tomlv ./*.toml
                                                                                                          
~/Projects/glean-contrib travis-ci  
❯ echo $?
0
```

cc @jonmagic