ref(tests): switch to example-dockerfile-http app

This application's final image size is significantly smaller than
example-go's, which should dramatically increase integration test times:

    REPOSITORY                                    TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
    10.132.162.4:5000/example-go                  v2                  363636be7289        3 minutes ago       957.1 MB
    10.132.162.4:5000/example-dockerfile-http     v2                  6b7c19477ae5        49 minutes ago      2.433 MB

Source at https://github.com/deis/example-dockerfile-http