Support required value as function

Author: alyyousuf7

lib/revalidator.js

@@ -249,7 +249,12 @@
     }
 
     if (value === undefined) {
-      if (schema.required && schema.type !== 'any') {
+      var isRequired = schema.required;
+      if (typeof isRequired === 'function') {
+        isRequired = isRequired.bind(object)();
+      }
+
+      if (isRequired && schema.type !== 'any') {
         return error('required', property, undefined, schema, errors);
       } else {
         return;