Upgrade to safe_yaml

[DavidAllison]

Hi @dkubb! Via twitter, @solnic suggested I write to you about whether you'd accept a PR to the 1.2 release branch of dm-types to upgrade the version of safe_yaml in use. Would you?

It looks like in the master branch of dm_types, safe_yaml is no longer explicitly specified in the gemfile. Not sure why offhand.

I'm really only concerned with the 1.2 release branch, however, and am trying to see if we can resolve the dependency problems between DM and RailsAdmin, which includes a dependency which itself includes the latest safe_yaml