Add better-npm-audit to check production packages

This is the same as [^1].

[^1]: https://github.com/alphagov/document-download-frontend/pull/120

package.json

@@ -34,19 +34,20 @@
   "devDependencies": {
     "@babel/core": "7.4.0",
     "@babel/preset-env": "7.4.2",
+    "better-npm-audit": "^3.7.3",
     "gulp": "4.0.2",
     "gulp-add-src": "1.0.0",
     "gulp-babel": "8.0.0",
     "gulp-better-rollup": "4.0.1",
     "gulp-clean-css": "4.3.0",
     "gulp-concat": "2.6.1",
-    "gulp-include": "2.4.1",
-    "gulp-sass": "5.0.0",
-    "gulp-uglify": "3.0.2",
     "gulp-css-url-adjuster": "0.2.3",
+    "gulp-include": "2.4.1",
     "gulp-jshint": "2.1.0",
     "gulp-prettyerror": "2.0.0",
+    "gulp-sass": "5.0.0",
     "gulp-sass-lint": "1.4.0",
+    "gulp-uglify": "3.0.2",
     "jest": "24.7.1",
     "jest-date-mock": "^1.0.8",
     "jest-each": "^25.3.0",