Added :type to ActiveFedora::QualifiedDublinCoreDatastream::DCTERMS.

Presumably this field was excluded due to Object#type from Ruby 1.8.
Since AF currently requires >= Ruby 1.9.3, it appears safe to add this DC 1.1 element.

Note: https://github.com/projecthydra/active_fedora/pull/135 is still relevant for :format at least, and possibly for other purposes.