Pin npm-shrinkwrap to v5.0.0 so that it works with npm 2.x.

Fixes #15105.

package.json

@@ -56,7 +56,7 @@
     "grunt-saucelabs": "~8.3.2",
     "grunt-sed": "~0.1.1",
     "load-grunt-tasks": "~1.0.0",
-    "npm-shrinkwrap": "~5.1.0",
+    "npm-shrinkwrap": "5.0.0",
     "remarkable": "~1.4.0",
     "time-grunt": "~1.0.0"
   },