Support transitive jar version forcing via <overrides/>.

This code change was missed in the merge from Twitter -> twitter/commons.  The corresponding template support was not missed - see: https://github.com/pantsbuild/pants/blob/master/src/python/pants/tasks/templates/ivy_resolve/ivy.mustache#L77
That bit of the template was just not exercised.

https://github.com/pantsbuild/pants/issues/39
https://rbcommons.com/s/twitter/r/198/