Core has a dependency on Shared.
Therefore, Shared cannot depend on the current snapshot of Core or we will have a circular dependency. (Note that it's in a plugin execution and not a normal <dependency>, so Maven won't catch it during dependency resolution.)
The JSF 1.1 API hasn't changed, so it shouldn't matter whether myfaces-impl builds against 1.1.1 or 1.1.5-SNAPSHOT (or against the API jar from the reference implementation, for that matter.) I'm not as opposed to this one, but I don't think it's necessary... and the fewer snapshot dependencies the better. The situation may improve once 1.1.4 is available, because it will be in the same groupId as the current snapshot.