Switch Precommit builds to Java 7

[Brock Noland]

Java 6 has been EOL for almost a year now and 7 is EOL in a short year and
a quarter[1]. Therefore, I think it's time to switch the Precommit builds
to Java 7.

Brock

1. http://www.oracle.com/technetwork/java/eol-135779.html