some cleanups to the meta and test suite to work with latest Dassets

No new behavior here - just cleanups.  These were never done once
we re-implemented Dassets source handling.

I also removed the system tests.  They were breaking, but obviously,
are not critical to the gem b/c the gem is working fine with the
latest Dassets.  Ultimately, these system tests were testing things
Dassets is responsible for testing.

@jcredding just like on dassets-sass - ready for review.