Enable rbtrace for unicorn if ENABLE_RBTRACE is set

rbtrace was added in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20831, but it only worked for Sidekiq because the unicorn master resets any signal handlers that are set during the Rails initializers. Moving the require 'rbtrace' calls to after the fork ensures that the rbtrace listener works for unicorn.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20892 as well.