(#19304) Upgrade to json_pure 1.7.7 for CVE-2013-0269

CVE-2013-0269 Denial of Service and Unsafe Object Creation Vulnerability in JSON

This is resolved for the dashboard by upgrading to json_pure 1.7.7 in our
vendored set of gems.  All our uses of the JSON API use the safe mechanisms
that this upgrade will ensure can't misbehave in fundamental ways.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>