README: Mustermann does not depend on tool

Since #49

Rubinius is dead. It is enough to test one JRuby until support has been
confirmed.

.github/workflows/test.yml

@@ -19,8 +19,7 @@ jobs:
         # Due to https://github.com/actions/runner/issues/849, we have to use quotes for '3.0'
         ruby: [2.6, 2.7, '3.0', 3.1, 3.2, 3.3, head, truffleruby]
         include:
-          - { ruby: jruby-9.3, allow-failure: true }
-          - { ruby: jruby-9.4, allow-failure: true }
+          - { ruby: jruby, allow-failure: true }
     steps:
     - uses: actions/checkout@v3
     - uses: ruby/setup-ruby@v1

README.md

@@ -164,9 +164,9 @@ Any software using Mustermann is obviously compatible with at least one of the a
 
 ## Requirements
 
-Mustermann depends on [tool](https://github.com/rkh/tool) (which has been extracted from Mustermann and Sinatra 2.0), and a Ruby 2.6+ compatible Ruby implementation.
+Ruby 2.6+ compatible Ruby implementation.
 
-JRuby and Rubinius support is unknown.
+JRuby support is unknown, see more in [issue #72](https://github.com/sinatra/mustermann/issues/72).
 
 ## Release History