Scala 2.11 compatibile build

 - Bump Scalacheck version (this is binary incompatible
   and should really result in non-minor version bump of
   Spire itself, as it has a module with scalacheck as a
   transitive dependency.)
 - Bump Scalatest version for 2.11

This leads to an apparent compiler regression:

    [error] /Users/jason/code/spire/core/src/main/scala/spire/math/Complex.scala:462: reference to sqrt is ambiguous;
    [error] it is both defined in object FastComplex and imported subsequently by
    [error] import java.lang.Math.{atan2, cos, sin, sqrt}
    [error]     sqrt(re * re + im * im).toFloat
    [error]     ^
    [error] one error found