Support testing in node 6.

- Use babel on the test code itself.

Author: davidlehn

tests/test.js

@@ -20,6 +20,14 @@
  *
  * Copyright (c) 2011-2017 Digital Bazaar, Inc. All rights reserved.
  */
+
+// support async/await tests in node6
+if(!require('semver').gte(process.version, '8.6.0')) {
+  require('babel-register')({
+    presets: ['node6-es6']
+  });
+}
+
 const assert = require('chai').assert;
 const common = require('./test-common');
 const fs = require('fs-extra');