Try building with Java 8 using jruby-gradle-plugin:2.0.2

.github/workflows/build.yml

@@ -68,7 +68,7 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest]
-        java: [ 11 ]
+        java: [ 8, 11 ]
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v1

docs/build.gradle

@@ -5,7 +5,8 @@ buildscript {
     }
     dependencies {
         /* wow. so recursion */
-        classpath "com.github.jruby-gradle:jruby-gradle-plugin:2.1.0-alpha.2"
+        classpath "com.github.jruby-gradle:jruby-gradle-plugin:2.0.2"
+        classpath 'org.ysb33r.gradle:grolifant:0.17.0'
     }
 }
 apply plugin: 'groovy'