Defaults Helper Has Incorrect Values on Initial Install if Bitcoin Core is Updated

The Helpers/Default.swift contains initial values for version and other information.

On an initial install, if Gordian Server discovers a new version of Bitcoin Core, the Default values do *not* update correctly. Instead, you have to restart the Server before they do. (I suspect they may happen in other instances where Gordian Server discovers a new version of Bitcoin Core, but I have only seen it on a fresh install.)

The situation is as follows:

1. Gordian Server discovers a new version of Bitcoin Core.
2. Gordian Server correctly downloads and installs the new version.
3. The scripts to start Bitcoin running and check for Bitcoin running, which depend on a `$PREFIX` variable look for the old version and so fail. Gordian Server is non-functional.
4. Gordian Server must be quit and restarted.
5. After the restart, everything now correctly looks for the new version rather than the old version.