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.