Spurious failure with 'test_massive_task_creation' test

[frewsxcv]

https://github.com/frewsxcv/rust-threadpool/blob/f8b13ee8a00386e63d9d430a5f9612bb51fd8b93/lib.rs#L553

example of failure: https://travis-ci.org/frewsxcv/rust-threadpool/jobs/251542770 via #64

---- test::test_massive_task_creation stdout ----

	thread 'test::test_massive_task_creation' panicked at 'atomic_active_count: 2', lib.rs:553:8

[bblancha]

When #63 gets merged, we should be able use use it to eliminate the race.

https://github.com/bblancha/rust-threadpool/blob/massive_task_creation_fix/lib.rs#L599

[frewsxcv]

#63 has merged