Use OpenJDK 7 instead of OpenJDK 6 with Gerrit.

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

Change-Id: Id5867a0269bc6af3e7f6214112e91c8848ffbbe4
Merge "Use OpenJDK 7 instead of OpenJDK 6 with Gerrit."