[SECURITY] Update Rack to latest stable on 1.2.x branch

Addresses a timing attack, which could
allow for remote code execution. References:
https://bugzilla.redhat.com/show_bug.cgi?id=909071
https://github.com/rack/rack/commit/0cd7e9aa397f8ebb3b8481d67dbac8b4863a7f07