Bump representable dependency to 3.0 (#532)

Also fixes a breaking change introduced between `representable` 2.3 and
3.0. Specifically, `:skip_undefined` is now be passed via the
`:user_options` option.

lib/google/apis/core/api_command.rb

@@ -55,7 +55,7 @@ module Google
           query[FIELDS_PARAM] = normalize_fields_param(query[FIELDS_PARAM]) if query.key?(FIELDS_PARAM)
           if request_representation && request_object
             header[:content_type] ||= JSON_CONTENT_TYPE
-            self.body = request_representation.new(request_object).to_json(skip_undefined: true)
+            self.body = request_representation.new(request_object).to_json(user_options: { skip_undefined: true })
           end
           super
         end