Update vocabulary based on recent RDFa context update

[gkellogg]

Update vocabulary based on recent RDFa context update:

• 'as' for the Activity Vocabulary (https://www.w3.org/ns/activitystreams#)
• 'dqv' for the Data Quality Vocabulary (http://www.w3.org/ns/dqv#)
• 'duv' for the Dataset Usage Vocabulary (https://www.w3.org/TR/vocab-duv#)
• 'ldp' for the Linked Data Platform Vocabulary (http://www.w3.org/ns/ldp#)
• 'oa' for the Web Annotation Vocabulary (http://www.w3.org/ns/oa#) (already present)