Fix build.gradle to avoid gem creation error on JDK8 / 11

Running ./gradlew gem raised an error relating to the use of  `dependsOn` that can't find the correct `getAt` method for a list argument

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")
     }
-}
+}