Revert "Change to Maven 3.3.9"

This reverts commit 3fa709a2d01ff687faa1d855130f5e30526439c3.
diff --git a/Jenkinsfile b/Jenkinsfile
index 3069f19..01a3e7e 100644
--- a/Jenkinsfile
+++ b/Jenkinsfile
@@ -8,7 +8,7 @@
 spec:
   containers:
   - name: maven
-    image: maven:3.3.9-jdk-8
+    image: maven:3.6.1-jdk-8
     command:
     - cat
     tty: true