fix: package.json to reduce vulnerabilities

The following vulnerabilities are fixed with an upgrade:
- https://snyk.io/vuln/SNYK-JS-ANSIREGEX-1583908
- https://snyk.io/vuln/SNYK-JS-GLOBPARENT-1016905
- https://snyk.io/vuln/SNYK-JS-INFLIGHT-6095116
- https://snyk.io/vuln/SNYK-JS-UNSETVALUE-2400660

package.json

@@ -4,7 +4,7 @@
     "babel-core": "^6.26.3",
     "babel-preset-es2015": "^6.24.1",
     "browser-sync": "^3.0.0",
-    "gulp": "^4.0.2",
+    "gulp": "^5.0.0",
     "gulp-concat": "^2.6.1",
     "gulp-postcss": "^10.0.0",
     "gulp-run-command": "^0.0.10",