Upgrade to scalacheck 1.12.0

specs2 is preventing us from upgrading to scalacheck 1.12.0 which is in turn preventing us from upgrading to scala 2.11.4. So this is my attempt at upgrading the scalacheck dependency.

The "A specification can reference itself without creating a loop" test in `FragmentsBuilderIntegrationSpec` is failing and I don't know enough about the changes to Scalacheck to fix it presently, but I'm going to seek help. In the meantime I am submitting this PR for commentary.