add JDK 13 to automated builds

Test all current LTS versions (8, 11) and the latest stable release (13)

.drone.yml

@@ -16,3 +16,13 @@ steps:
     image: maven:3-jdk-11-slim
     commands:
       - mvn clean test
+
+---
+kind: pipeline
+name: java13
+
+steps:
+  - name: test
+    image: maven:3-jdk-13
+    commands:
+      - mvn clean test

.travis.yml

@@ -2,6 +2,7 @@ language: java
 jdk:
   - openjdk8
   - openjdk11
+  - openjdk13
 install: true
 addons:
   sonarcloud: