Improve work-around for importScripts bug.

Reverts "Hack to avoid intermidiate Chrome failures during tests."
(2b2c521).

Updates require.js to the latest version (pre-release), which includes
an alternative work-around for the bug that was worked around by the
reverted commit. The advantage of the new work-around is that it allows
the runtime to garbage-collect idle Workers.

References:
- https://crbug.com/572225
- https://webkit.org/b/153317
- requirejs/requirejs#1469

package.json

@@ -5,7 +5,7 @@
     "jsdoc": "^3.3.0-alpha9",
     "jshint": "~2.8.0",
     "node-ensure": "^0.0.0",
-    "requirejs": "^2.1.22",
+    "requirejs": "jrburke/requirejs#a1da39714bc3eea9ab85c77e47d544e815e2699f",
     "rimraf": "^2.4.1",
     "shelljs": "~0.4.0",
     "typogr": "~0.6.5",

src/display/api.js

@@ -1276,16 +1276,7 @@ var PDFWorker = (function PDFWorkerClosure() {
           // https://bugzilla.mozilla.org/show_bug.cgi?id=683280
           var worker = new Worker(workerSrc);
           var messageHandler = new MessageHandler('main', 'worker', worker);
-//#if !PRODUCTION
-          // Don't allow worker to be destroyed by Chrome, see:
-          // https://code.google.com/p/chromium/issues/detail?id=572225
-          var jsWorkerId = '_workerKungfuGrip_' + Math.random();
-          window[jsWorkerId] = worker;
-//#endif
           messageHandler.on('test', function PDFWorker_test(data) {
-//#if !PRODUCTION
-            delete window[jsWorkerId];
-//#endif
             if (this.destroyed) {
               this._readyCapability.reject(new Error('Worker was destroyed'));
               messageHandler.destroy();