Use OpenJDK 7 instead of OpenJDK 6 with Jenkins.

Oracle has EOLed Java 6. While OpenJDK 6 is still supported, development
on it has slowed. Upgrade to OpenJDK 7 and run Jenkins on this newer
platform.

Change-Id: I01c90133c6bc4f2ae0a25e35e5f8a11dc96ebd0e
Merge "Use OpenJDK 7 instead of OpenJDK 6 with Jenkins."