Bump to our own nightly of ivy 2.4.0 (RC1+).

* Testing the infrastructure to use our own instance of ivy
* Allow us to push bug-fixes/performance directly into ivy
  on our own pace, as we submit patches back to ivy master
Bump ivy version to include exclusion performance fix.
Merge pull request #1226 from sbt/wip/bump-to-latest-ivy

Bump to our own release of ivy 2.4.x