Update Jenkins setup instructions for custom ec2-plugin

Summary:
We added a change in our fork so that a timeout of zero really does mean
"try forever". See https://github.com/Khan/ec2-plugin/commit/d236a8c5aa72f2e3adec1ae325edfd27488bf896

Test Plan:
Ran the equivalent of the steps (I actually ran them locally, relying on
the HipChat plugin instructions working w/Maven) and verified the new
plugin version was installed to Jenkins.

Reviewers: csilvers

Reviewed By: csilvers

Differential Revision: http://phabricator.khanacademy.org/D10553