Add Maven Central to registeredRepositories

Add Maven Central to registeredRepositories

Moxie does have these defined by default, but with transport HTTP.
Since January 2020, Maven Central only accepts HTTPS, no HTTP
anymore. So we add them here explicitly, with `https://`.
We add it to the `build.moxie` file, instead of keeping it in a
local `settings.moxie`, so that a CI server can immediately use it.