Allow normalize-package-data@^2.0.0

Not forcing 2.0.0 as we don't rely on any of it's features nor
breaking changes.

Author: iarna

package.json

@@ -15,7 +15,7 @@
     "concat-stream": "^1.4.6",
     "graceful-fs": "^3.0.0",
     "mkdirp": "^0.5.0",
-    "normalize-package-data": "~1.0.1",
+    "normalize-package-data": "~1.0.1 || ^2.0.0",
     "npm-package-arg": "^3.0.0",
     "once": "^1.3.0",
     "request": "^2.47.0",