Build and test on both AMD64 and ARM64

Use 'arm64' instead of 'arm64-graviton2'

The Graviton2 builder is available only on travis-ci.com. Apache uses travis-ci.org

Install Maven explicitly because it is not pre-installed on the ARM64 image for Trusty

.travis.yml

@@ -17,6 +17,15 @@ language: java
 dist: trusty
 sudo: false
 
+arch:
+  - amd64
+  - arm64
+
+addons:
+  apt:
+    packages:
+      - maven
+      
 jdk:
   - oraclejdk8
   - openjdk12