Bump to our own release of ivy 2.4.x

* Bumps to our own branch of ivy (github.com/sbt/ivy)
* Includes a ~50% improvement in performance when dealing
  with maven resolution + dependencyManagement fun
* Tweaks to API differences between Ivy 2.3 + 2.4

Review by @eed3si9n 

cc> @Kewpie007