Update README with latest tested environment

Author: carlosefr

README.md

@@ -15,7 +15,7 @@ It's a Clojure application so you'll need:
 * [Java](http://jdk.java.net/) (OpenJDK is recommended)
 * [Leiningen](https://leiningen.org/)
 
-It was last known to work with OpenJDK 13 on macOS 10.13.6.
+It was last known to work with OpenJDK 15.0.1 on macOS 10.15.7.
 
 Once you have these requirements installed, run `lein run` in the project directory. It will download the necessary dependencies and start an HTTP server on port `tcp/3000`.