Change the update mechanism so it is driven by the version of sovrin.deb, not the version of indy-node.deb

We change the update mechanism so it is driven by the version of sovrin.deb, not the version of indy-node.deb. This means that any time we want to update the sovrin network, we build a new sovrin.deb that changes the dependency that it declares, on indy-node. (We almost certainly will have to update indy-node as well, of course.)