Updates rack to prevent security vuln

Updates rack and a few other Rails dependencies to patch against:

CVE-2013-0263: timing attack against Rack::Session::Cookie 
CVE-2013-0262: symlink path traversal in Rack::File 

Source: https://groups.google.com/forum/#!msg/rack-devel/bf937jPZxJM/1s6x95vIhmAJ