fix(yarn): add missing requirement

Author: mlunoe

package.json

@@ -17,7 +17,8 @@
     "postversion": "git push origin refs/tags/v\"$npm_package_version\" && git push && echo \"✨ Successfully released version v$npm_package_version! The tag build process will publish it ✨\""
   },
   "engines": {
-    "node": ">=8.9.3"
+    "node": ">=8.9.3",
+    "yarn": ">=1.9.4"
   },
   "gh-pages-deploy": {
     "staticpath": "docs",