Prepare upgrade to scalacheck 1.11.

Our scalacheck tests now compile against 1.10.1 and 1.11.0.
They pass on 1.10.1, but fail on 1.11.0.

Once (that)[https://github.com/rickynils/scalacheck/issues/79]'s fixed,
and 1.11.1 released, we should be able to upgrade to it by simply
changing scalacheck.version.number in versions.properties.

The changes are mostly removing dead code (e.g., consolereporter business).

Of interest: the type ascription for `oneOf`. I haven't quite investigated,
but something seems to have changed between 1.10.1 and 1.11.0 that caused
a different overload to be picked without the type ascription.
Probably not a scalac bug, just a scalacheck api change.