tests: run BrowserStack in parallel with Sauce Labs

Per https://github.com/twbs/bootstrap/issues/11087#issuecomment-31922999
and internal twbs team discussion.
Effectively reverts the deletions from #11701.

package.json

@@ -25,6 +25,7 @@
     }
   ],
   "devDependencies": {
+    "browserstack-runner": "~0.0.13",
     "btoa": "~1.1.1",
     "grunt": "~0.4.2",
     "grunt-banner": "~0.2.0",