Bump Rack version in Gemfile.lock

This fixes the recently announced vulnerabilities in Rack for a timing attack on sessions and a subsequent remote code execution vulnerability (CVE-2013-0263) as well as a path traversal vulnerability (CVE-2013-0262).

All of these vulnerabilities are in Rack only which Kibana uses as a dependency through Sinatra.

As a side-remark: I upgraded rack conservatively. Sinatra should be able to also support Rack 1.5.2. But as I don't know what other dependencies expect, I though to play it safe :)