fix: artifact-version needs to use Manifest#getMainAttributes

getAttributes returns null

artifact-version.groovy

@@ -8,7 +8,7 @@ Manifest manifest = new Manifest(
   new FileInputStream(
     new File("${build.workspace}/build/tmp/jar/MANIFEST.MF")));
 
-def version = manifest.getAttributes().getValue(Attributes.Name.IMPLEMENTATION_VERSION)
+def version = manifest.getMainAttributes().getValue(Attributes.Name.IMPLEMENTATION_VERSION)
 println "Artifact version is $version"
 build.addAction(
   new ParametersAction([

build.gradle

 group 'edu.wisc.doit.gradle'
-version '0.2.0'
+version '0.2.1'
 
 apply plugin: 'groovy'
 apply plugin: 'idea'