Automatically find the latest stable version

Author: eregon

.github/workflows/test.yml

@@ -6,7 +6,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ 'ubuntu-16.04', 'ubuntu-18.04', 'macos-latest' ]
-        ruby: [ 'ruby-2.6.5', 'ruby-2.7.0', 'truffleruby-19.3.0', 'jruby-9.2.9.0' ]
+        ruby: [ 'ruby-2.6.5', '2.7.0', 'truffleruby', 'jruby-9.2.9.0' ]
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2

README.md

@@ -46,6 +46,10 @@ jobs:
     - run: ruby -v
 ```
 
+If a specific version is not given, it uses the latest stable release of that implementation.
+
+For instance `truffleruby` is currently the same as `truffleruby-19.3.0`.
+
 ## Efficiency
 
 It takes about 5 seconds to setup the given Ruby.