Upgrade Rack version

Summary:
This is a security fix to force rack 2.0.6.

References:

• https://bugzilla.redhat.com/show_bug.cgi?id=1646814 (CVE-2018-16470)
• https://bugzilla.redhat.com/show_bug.cgi?id=1646818 (CVE-2018-16471)

Test Plan: Tested against https://ci.nasqueron.org

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2059