Bump scalding from 0.9.0rc4 to 0.9.0rc15

* Add scalding-json which is now separate package
* Needs word -- Fix unit tests involving JsonLine, they don't
  actually apply the JSON transformation in test mode
  unless you force it, see
  https://github.com/twitter/scalding/blob/5663ff65d44e5547cf700f69c908f722fdff7739/scalding-core/src/main/scala/com/twitter/scalding/Source.scala#L91
* remove with TupleConversions usage since it's now deprecated/implicit