Add an OkJson backend and remove the YAML backend

Fixes CVE-2013-0333.  The ActiveSupport::JSON::Backends::Yaml class is present but the functionality has been removed entirely.
Merge pull request #172 from haus/cve-2013-0333

Add an OkJson backend and remove the YAML backend
Update VERSION to 1.2.21
Merge branch '1.2rc'