vendorise upgradeable_http
Make application exit when listeners return

Necessary so that the temporary child will exit when reloading.
Set upgradeable_http.WorkingDir from GOVUK_APP_ROOT

This will allow restarts to pick up the new version of the application
when symlinked directories are updated.  See
http://godoc.org/github.com/alext/upgradeable_http#pkg-variables for
more details