Fix GenGen compile errors.

TODO: Try to find a smoother upgrade path---compile errors would likely not indicate to a user that this change was required.  Example error message:
[error] /Users/broberts/Documents/workspace/scalatest/gentests/GenGen/target/scala-2.11/src_managed/test/scala/gengen/GeneratorDrivenPropertyChecksMixinSuite.scala:702: could not find implicit value for parameter config: GeneratorDrivenPropertyChecksMixinSuite.this.PropertyCheckConfiguration
[error]     forAll { (a: String) =>
[error]            ^