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).
`$GOPATH` is set to `/home/travis/gopath:/home/travis/.gvm/pkgsets/go1.3.3/global`, and `/home/travis/.gvm/pkgsets/go1.3.3/global/bin` is on `$PATH`.