Update gitlab-grack to 2.0.1 to get unicorn to stop creating zombies.

Addresses #1461 and private issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2253.

This regression was introduced between 7.9 and the 7.10 rcs, so no changelog item for 7.10 proper.

cc @JobV @jacobvosmaer

See merge request !546