Upgrade @tryghost/validator

no issue
Fixes the build - as test checks for instanceof ValidationError, and
we were using 2 versions of @tryghost/errors between Ghost and
@tryghost/validator.

package.json

@@ -77,7 +77,7 @@
     "@tryghost/tpl": "0.1.2",
     "@tryghost/update-check-service": "0.1.0",
     "@tryghost/url-utils": "1.1.4",
-    "@tryghost/validator": "0.1.0",
+    "@tryghost/validator": "0.1.1",
     "@tryghost/version": "0.1.0",
     "@tryghost/vhost-middleware": "1.0.15",
     "@tryghost/zip": "1.1.14",
@@ -107,6 +107,7 @@
     "glob": "7.1.7",
     "got": "9.6.0",
     "gscan": "4.1.0",
+    "gulp": "^4.0.2",
     "html-to-text": "5.1.1",
     "image-size": "1.0.0",
     "intl": "1.2.5",