Use kind-of >=6.0.3 everywhere

This repo's dependencies currently use [older versions](https://github.com/paritytech/operationsproxy/blob/1e42f36ab60f6368f673211d53e9b887227dc11d/yarn.lock#L3521) of kind-of which have a possibly exploitable vulnerability ([original issue](jonschlinkert/kind-of#30), [PR](jonschlinkert/kind-of#31)).

package.json

@@ -22,6 +22,9 @@
     "webpack-cli": "3.3.10",
     "webpack-dev-server": "3.10.1"
   },
+  "resolutions": {
+    "kind-of": ">=6.0.3"
+  },
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1",
     "lint": "semistandard",