Update annotation.rb

According to http://iiif.io/api/presentation/2.0/#image-resources, "the URI of the canvas must be repeated in the on field of the Annotation". This patch adds the "on" field to the annotation class and makes it a required field. 

It's worth noting that without this patch, https://github.com/IIIF/presentation-validator will fail the validation for an O'Sullivan generated manifest that contains an annotation. With the patch, it will succeed.

lib/iiif/presentation/annotation.rb

@@ -7,9 +7,12 @@ class Annotation < AbstractResource
       TYPE = 'oa:Annotation'
 
       def required_keys
-        super + %w{ motivation }
+        super + %w{ motivation on }
       end
 
+      def string_only_keys
+        super + %w{ on }
+
       def abstract_resource_only_keys
         super + [ { key: 'resource', type: IIIF::Presentation::Resource } ]
       end