fix: package.json to reduce vulnerabilities (#4449)

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:hoek:20180212

Latest report for mozilla/addons-frontend:
https://snyk.io/test/github/mozilla/addons-frontend

package.json

@@ -255,7 +255,7 @@
     "mock-express-request": "^0.2.0",
     "mock-express-response": "^0.2.0",
     "node-sass": "^4.5.2",
-    "nsp": "^2.7.0",
+    "nsp": "^3.0.0",
     "object.values": "^1.0.4",
     "photon-colors": "^1.0.3",
     "piping": "^1.0.0-rc.4",