bump grunt-saucelabs to ~8.4.0

[skip validator]

package.json

@@ -54,7 +54,7 @@
     "grunt-html-validation": "~0.1.18",
     "grunt-jekyll": "~0.4.2",
     "grunt-jscs": "~1.0.0",
-    "grunt-saucelabs": "~8.3.3",
+    "grunt-saucelabs": "~8.4.0",
     "grunt-sed": "~0.1.1",
     "load-grunt-tasks": "~1.0.0",
     "npm-shrinkwrap": "^200.0.0",