DATAFU-157 Add support for newer versions of Gradle

This commit updates the autojar to
reflect the changes to the Gradle 5
API.

Signed-off-by: Eyal Allweil <eyal@apache.org>
diff --git a/buildSrc/build.gradle b/buildSrc/build.gradle
index 02c21af..cd2915f 100644
--- a/buildSrc/build.gradle
+++ b/buildSrc/build.gradle
@@ -22,7 +22,7 @@
 }
 
 dependencies {
-  compile 'com.github.rholder:gradle-autojar:1.0.1'
+  implementation 'com.github.rholder:gradle-autojar:1.0.1'
 }
 
 test.enabled=false
diff --git a/buildSrc/src/main/groovy/datafu/autojar/task/Autojar.groovy b/buildSrc/src/main/groovy/datafu/autojar/task/Autojar.groovy
index a620337..dd382df 100644
--- a/buildSrc/src/main/groovy/datafu/autojar/task/Autojar.groovy
+++ b/buildSrc/src/main/groovy/datafu/autojar/task/Autojar.groovy
@@ -23,6 +23,7 @@
 import org.gradle.api.java.archives.Manifest
 import org.gradle.api.java.archives.internal.DefaultManifest
 import org.gradle.api.logging.Logger
+import org.gradle.api.provider.Property
 import org.gradle.api.tasks.JavaExec
 import org.gradle.api.tasks.TaskAction
 import org.gradle.api.tasks.TaskDependency
@@ -41,7 +42,7 @@
 
     Configuration targetConfiguration
 
-    String mainClass
+    Property<String> mainClass
     List<String> autojarClasses    // convert these to raw files
     List<String> autojarFiles      // all the class files, etc.