Drop Hudson compatabilty.
Upgrade Jenkins HPI plugin.
Update staging to work with Nexus 2.4+

Nexus 2.4 introduced aynchronous notifications for staging in a
non backwards compatable way.

This adds support for the new way but uses does not enable it if the
server version is < 2.4.

Change-Id: I168214d824be094c7bcfac50b29d4dc6e014fd67