Update to use QUnit 2.11 and require Node.js 10 or later

Per <https://github.com/nodejs/Release>, Node 6 and 8 have
reached EOL in 2019.

The latest QUnit removed support for Node.js 8 and earlier,
so to use it node-qunit has to do this as well.

Commit package-lock.json to ensure local development and CI
behaviour is more deterministic, and slightly faster.

Author: Krinkle

.travis.yml

@@ -1,5 +1,5 @@
 language: node_js
 node_js:
+  - 14
+  - 12
   - 10
-  - 8
-  - 6