Update jdk14 and build with openjdk15,16
diff --git a/.travis.yml b/.travis.yml
index 73c2f8f..c3fc179 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -11,7 +11,9 @@
     - env: DOCKER_IMAGE=maven:3.6.3-jdk-11
     - env: DOCKER_IMAGE=maven:3.6.2-jdk-12
     - env: DOCKER_IMAGE=maven:3.6.2-jdk-13
-    - env: DOCKER_IMAGE=maven:3.6.2-jdk-14
+    - env: DOCKER_IMAGE=maven:3.6.3-jdk-14
+    - env: DOCKER_IMAGE=maven:3.6.3-openjdk-15
+    - env: DOCKER_IMAGE=maven:3.6.3-openjdk-16
 
 install:
   - docker pull "${DOCKER_IMAGE}"