Update travis build vm dist form xenial to focal (#226)

* Update travis build vm dist form xenial to focal 
* use java11
diff --git a/.travis.yml b/.travis.yml
index ffaa53c..73ce94a 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -16,8 +16,8 @@
 #
 
 sudo: required
-dist: xenial
-jdk: openjdk8
+dist: focal
+jdk: openjdk11
 language: java
 services:
   - docker