Bump npm from 6.10.3 to 6.11.3

[npm 6.11.3 has been released](https://npm.community/t/release-6-11-3/9873/1) which fixes [the regression we've been seeing](https://npm.community/t/6-11-1-some-dependencies-are-no-longer-being-installed/9586/4).

Ref #3904

Author: paulmelnikow

package.json

@@ -256,7 +256,7 @@
   },
   "engines": {
     "node": "^10.16.3",
-    "npm": "6.10.3"
+    "npm": "^6.11.3"
   },
   "babel": {
     "plugins": [