Update README.txt location

Gitorious has changed its Web UI, so the URL needs an update.

Change-Id: Ic25c9269e9135ba3d6976a8ad7f11c0ce45bae8d
Reviewed-by: Iikka Eklund <iikka.eklund@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>