SignedXml - use the right Buffer ctor for digest compares

Do not rely on Buffer.from being a function, as Node 4 does not
support encoding with Buffer.from. RegExp the major version instead;
if 6 or greater, the ctor is deprecated, and we should use .from

Author: LoneRifle

lib/signed-xml.js

@@ -494,7 +494,9 @@ SignedXml.prototype.validateReferences = function(doc) {
 function validateDigestValue(digest, expectedDigest) {
   var buffer, expectedBuffer;
 
-  if (typeof Buffer.from === 'function') {
+  var majorVersion = /^v(\d+)/.exec(process.version)[1];
+
+  if (+majorVersion >= 6) {
     buffer = Buffer.from(digest, 'base64');
     expectedBuffer = Buffer.from(expectedDigest, 'base64');
   } else {