FIX: Manifest validation false positive for type: "web"

Manifest validation false positive for type: "web" (fix #351)

- appType - valid app types: web, privileged, certified
- appStatus - permission table lookup key: app, privileged, certified
  (If it isn't certified or privileged, it's appStatus "app")
  https://hg.mozilla.org/releases/mozilla-b2g18/file/d9278721eea1/dom/apps/src/PermissionsTable.jsm#l413