fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/npm:minimatch:20160620

Latest report for alphagov/notifications-admin:
https://snyk.io/test/github/alphagov/notifications-admin

package.json

@@ -25,7 +25,7 @@
     "govuk_frontend_toolkit": "7.2.0",
     "govuk_template_jinja": "0.23.0",
     "gulp": "3.9.1",
-    "gulp-add-src": "0.2.0",
+    "gulp-add-src": "1.0.0",
     "gulp-babel": "7.0.0",
     "gulp-base64": "0.1.3",
     "gulp-concat": "2.6.1",