Fix build.gradle to avoid execution error on JDK8 / 11

Running `./gradlew gem` on JDK 8 / 11 raises the error "No signature of method: org.gradle.api.internal.tasks.DefaultTaskDependency$TaskDependencySet.getAt()".
This maybe is due to API changes, this commit fixes the problem

Author: andsel

build.gradle

@@ -98,8 +98,8 @@ tasks.register("removeObsoleteJars") {
 }
 
 tasks.register("gem"){
-    dependsOn [downloadAndInstallJRuby, removeObsoleteJars, vendor, generateRubySupportFiles]
+    dependsOn = [downloadAndInstallJRuby, removeObsoleteJars, vendor, generateRubySupportFiles]
     doLast {
         buildGem(projectDir, buildDir, pluginInfo.pluginFullName() + ".gemspec")
     }
-}
+}