src: fix deprecated use of Buffer::New()

Pass the isolate explicitly.  Overlooked in commit ccb199a ("src: fix
deprecation warnings") because g++ 4.8 and 4.9 don't warn for it
whereas g++ 5.1 does.

PR-URL: #1577
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

src/string_bytes.cc

@@ -681,7 +681,7 @@ Local<Value> StringBytes::Encode(Isolate* isolate,
   Local<String> val;
   switch (encoding) {
     case BUFFER:
-      return scope.Escape(Buffer::New(buf, buflen));
+      return scope.Escape(Buffer::New(isolate, buf, buflen));
 
     case ASCII:
       if (contains_non_ascii(buf, buflen)) {