Updated NailgunExecutor to take the java version into account when deciding if the server needs to be re-started.

Testing Done:
CI passed @ https://travis-ci.org/pantsbuild/pants/builds/31273593

setjdk 1.7
pants goal bundle src/java/com/pants/examples/hello/main
ps auxww | grep NGServer  (all are running java 1.7)

setjdk 1.8
ants goal bundle src/java/com/pants/examples/hello/main
ps auxww | grep NGServer  (get info messages about NG servers killed.  All are now running java 1.8)

Bugs closed: 423

Reviewed at https://rbcommons.com/s/twitter/r/802/