update Drone CI config, switch to OpenJDK on Travis

.drone.yml

@@ -1,34 +1,18 @@
 kind: pipeline
 name: java8
 
-clone:
-  disable: true
-
 steps:
-- name: clone
-  image: plugins/git
-  settings:
-    depth: 10
-    skip_verify: true
-- name: test
-  image: maven:3-jdk-8-alpine
-  commands:
-  - mvn clean test
+  - name: test
+    image: maven:3-jdk-8-alpine
+    commands:
+      - mvn clean test
 
 ---
 kind: pipeline
 name: java11
 
-clone:
-  disable: true
-
 steps:
-- name: clone
-  image: plugins/git
-  settings:
-    depth: 10
-    skip_verify: true
-- name: test
-  image: maven:3-jdk-11-slim
-  commands:
-  - mvn clean test
+  - name: test
+    image: maven:3-jdk-11-slim
+    commands:
+      - mvn clean test