Bump eslint v2 -> v6

Primarily because it's been ages since we bumped eslint and as a precursor
to allowing some future test files to be written in a more modern type
of JavaScript. Especially think about tests written using puppeteer.js
which is heavily `Promise` based, where the async-await syntax makes
it soooo less painful to write.

Silly not being able to use that because of an outdated eslint version
doesn't understand that syntax construct at all.

package.json

@@ -41,7 +41,7 @@
   },
   "devDependencies": {
     "chai": "^3.4.0",
-    "eslint": "2.5.1",
+    "eslint": "^6.5.1",
     "jshint": "^2.9.5",
     "mocha": "^3.0.2",
     "uglify-js": "^3.4.6",