[Fix] restore node < 4 compat

Fixes #314.

Author: ljharb

lib/elliptic/utils.js

@@ -14,7 +14,9 @@ utils.encode = minUtils.encode;
 // Represent num in a w-NAF form
 function getNAF(num, w, bits) {
   var naf = new Array(Math.max(num.bitLength(), bits) + 1);
-  naf.fill(0);
+  for (var i = 0; i < naf.length; i += 1) {
+    naf[i] = 0;
+  }
 
   var ws = 1 << (w + 1);
   var k = num.clone();