Kill all nailgun tasks after an integration test run to keep from leaking NGServer instances.

Before this change, Travis-CI builds are failing with: Cannot create GC thread. Out of system resources.
For example:  https://travis-ci.org/pantsbuild/pants/jobs/40841557

Testing Done:
CI running at: https://travis-ci.org/pantsbuild/pants/builds/41295684

Manual test: while running devpants goal test ./tests/python/pants_test:integration

before:
ps auxww | grep NGServer | wc -l
143

after:
ps auxww | grep NGServer | wc -l
13

Bugs closed: 799

Reviewed at https://rbcommons.com/s/twitter/r/1345/
Sync of https://github.com/pantsbuild/pants/commit/a2416d44f5c55c8f496f70d3d707900fe1735d63