Bump rack gem to 2.0.7

This update has two important fixes:

1. It reverts the monkey patch introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23385 since
https://github.com/rack/rack/pull/1201 is now part of the release.

2. Preserve forwarded IP address for trusted proxy chains
(https://github.com/rack/rack/pull/1343).