Make groovy-bsf an optional "module" for 2.5.1+, i.e. not reference by groovy-all

The groovy-all dependency brings in all the common "modules" for Groovy. I don't think groovy-bsf is in common use any more. Plus the transitive bsf jar dependency isn't OSGi complaint which breaks* OSGi builds wanting to use the groovy-all dependency. Users needing bsf can simply add that dependency to their build.

 [*] Provides an annoying warning at least.