Avoid mixing two flavours of Guice on the classpath.

Note: the main blocker to using Google-Guice for Maven3 is the lack of a release with the new ProvisionListener API, which is needed to support Plexus lifecycles. While there are other fixes/features in Sisu-Guice that are nice-to-have (such as removal of the big global singleton lock) they aren't strictly required for Maven3.