Provide built-in Godep support

As suggested by Josh Kalderimis [here](http://b.z19r.com/post/testing-golang-apps-on-travisci-with-godep#comment-1641772550).
It should stay, but should not treat `$GOPATH` as singular, meaning it should be:

``` ruby
       set 'PATH', '$HOME/gopath/bin:$PATH'
```