Build: Update grunt-contrib-qunit to fix caching of Chromium binary

* Puppeteer stores the Chromium binary only in node_modules/
  (not in $XDG_CACHE_HOME or some other directory that is generally
  persisted in dev and CI environments, nor in any other directory
  that one can opt-in to caching/persistence).

  This was fixed in Puppeteer 5+ (ref puppeteer/puppeteer#6014),
  through an opt-in PUPPETEER_DOWNLOAD_PATH env var.

* I enalbed this in commit 7cd99a2, but it didn't do anything
  yet as grunt-contrib-qunit wasn't using the newer Puppeteer yet.

  This was fixed in gruntjs/grunt-contrib-qunit#173.

package.json

@@ -66,7 +66,7 @@
     "grunt": "^1.4.1",
     "grunt-contrib-connect": "^3.0.0",
     "grunt-contrib-copy": "^1.0.0",
-    "grunt-contrib-qunit": "^4.0.0",
+    "grunt-contrib-qunit": "^5.0.1",
     "grunt-git-authors": "^3.2.0",
     "grunt-search": "^0.1.8",
     "kleur": "4.1.4",