bump scalacheck version
Avoid SBT's "Not a simple type" warning during API extraction.

    [warn] Not a simple type:
    [warn]  Type: scalaz.Unapply[scalaz.Applicative,List[Int]]{type M[X] = List[X]; type A = Int} (class scala.reflect.internal.Types$RefinedType0)
    [warn]  Transformed: class xsbti.api.Structure
    [warn] Not a simple type:
    [warn]  Type: scalaz.Unapply[scalaz.Applicative,List[Int]]{type M[X] = List[X]; type A = Int} (class scala.reflect.internal.Types$RefinedType0)
    [warn]  Transformed: class xsbti.api.Structure

SBT spits out this warnings in 0.13.0 for the members `f` and `g`
of `FunctTest`. Moving them local, rather than members,
avoids this problem.

I think this problem is fixed in SBT 0.13.1 with:

  https://github.com/sbt/sbt/issues/559