Don't aggregate bfg-benchmark in the build, to permit it Java 7

The core BFG functionality is staying Java 6 compatible for the time being,
to maximise convenience for Mac OS X users (it's what Apple gives them
by default):

http://support.apple.com/kb/HT5945

However, I'd like to be able to use Java 7 features in bfg-benchmark...
removing it from aggregation means that I can still easily have Travis
running CI against Java 6 without the build failing due to unavailable
Java 7 features.