Remove special case for dm-core in use_orm
Remove special case for dm-core in use_orm

merb_datamapper is now maintained by the DataMapper folks, and they don't call
it dm-merb anymore.  Removed the resolve_orm_plugin_name() method and now
just use merb_#{orm}

merb/merb,wycats/merb,wycats/merb,benburkert/merb-core,merb/merb,wycats/merb,merb/merb,benburkert/merb-core,wycats/merb,wycats/merb