Fix installation of URL polyfill with js-polyfills v0.1.16

js-polyfills v0.1.15 used to install the polyfill on
`this.URL`, where `this` was set to module.exports in
the context of a Browserify bundle. Consequently the result
was available as `require('js-polyfills/url').URL`.

As of v0.1.16 however, it installs the polyfill on `self`,
which is the Window object in the context of the browser
and so `require('js-polyfills/url').URL` is undefined.

Fixes #3064

h/static/scripts/polyfills.js

@@ -9,7 +9,7 @@ require('core-js/fn/object/assign');
 try {
   new window.URL('https://hypothes.is');
 } catch (err) {
-  window.URL = require('js-polyfills/url').URL;
+  require('js-polyfills/url');
 }
 
 // document.evaluate() implementation,

package.json

@@ -44,7 +44,7 @@
     "inherits": "^2.0.1",
     "is-equal-shallow": "^0.1.3",
     "jquery": "1.11.1",
-    "js-polyfills": "^0.1.11",
+    "js-polyfills": "^0.1.16",
     "mkdirp": "^0.5.1",
     "ng-tags-input": "2.2.0",
     "node-uuid": "^1.4.3",