DELAYED: juju-core migration to github

[Ian Booth]

Hi all

The migration to github was scheduled for 10:00 UTC Thursday 29 May.
Due a few reasons, including CI highlighting some critical regressions as per
Curtis' recent email, it is too risky to pull the trigger at the scheduled time.

The new cutover date will be 10:00 UTC Tuesday 3 June.

As before, please ensure as much work as possible is finalised and committed to
trunk before that time to reduce the amount of work you'll have to do to bring
across work in progress after the cutover.