Fix java_agent

Testing Done:
yes locally,

[tw-mbp13-tdesai birdcage]$ head util/util-class-preloader/src/main/java/BUILD
java_agent(name='java',
  sources=rglobs('*.java'),
  premain='com.twitter.classpreloader.ClassPreloader',
  can_retransform=True
)
[tw-mbp13-tdesai birdcage]$

[tw-mbp13-tdesai birdcage]$ ./pants goal list ./util/util-class-preloader/src/main/java/::
util/util-class-preloader/src/main/java/BUILD:java
[tw-mbp13-tdesai birdcage]$

Bugs closed: 217

Reviewed at https://rbcommons.com/s/twitter/r/506/