Silence deprecation

Warning was:
```
DEPRECATION WARNING: to_class_uri is deprecated and will be removed from
a future release (use 'to_rdf_representation()' instead).
called from model_class_uri at
/home/travis/build/projecthydra/curation_concerns/app/search_builders/curation_concerns/collection_search_builder.rb:19
```

app/search_builders/curation_concerns/collection_search_builder.rb

@@ -16,7 +16,7 @@ def initialize(scope, access = nil)
 
     # @return [String] class URI of the model, as indexed in Solr has_model_ssim field
     def model_class_uri
-      ::Collection.to_class_uri
+      ::Collection.to_rdf_representation
     end
 
     # @return [String] Solr field name indicating default sort order