Install OpenJDK 8 from the Jessie backports

Summary:
Jenkins now requires slaves to run JDK 8.

Fixes T1282.

Test Plan: Build an image for a ci.nasqueron.org slave container, run a PHP task in it.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1282

Differential Revision: https://devcentral.nasqueron.org/D1104