Use arm64-graviton2 on Travis (#7016)

Author: janaknat

.travis.yml

@@ -69,7 +69,10 @@ env:
 jobs:
     include:
         - env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 ARM64=1
-          arch: arm64
+          arch: arm64-graviton2
+          dist: bionic
+          virt: lxd
+          group: edge
           if: type = cron
         - env: ENABLE_ZTS=1 ENABLE_DEBUG=1 SKIP_IO_CAPTURE_TESTS=1 S390X=1
           arch: s390x