Update test case actually use the custom KryoInjection.

I recently submitted a pull request that was merged (https://github.com/twitter/chill/pull/51).  Thank you for the vote of confidence.  However, I didn't have it quite right.  I didn't realize that Kryo will serialize objects that don't have a serializer registered because this does *not work on the scala console*.

    > import com.twitter.chill._
    > case class Point(x: Int, y: Int)
    > KryoInjection.invert(KryoInjection(Point(4, 5)))
    res0: Option[java.lang.Object] = None
    > KryoBijection.invert(KryoBijection(Point(4, 5)))
    com.esotericsoftware.kryo.KryoException: Unable to find class: Point

My test case worked and I figured it could only work with my custom `KryoInjection `class.  Not so--since the case classes were not embedded inside another class they could use Kryo's default serialization.  I moved these case classes inside the Spec and removed the misleading comment I had copied about how the case classes "should be outside KryoSpec".

The test case still passes, but it fails if I revert from `MyKryoInjection` to `KryoInjection`.