ci: update arm64 Travis jobs to Graviton2

ci: update arm64 Travis jobs to Graviton2

Use only the newer Graviton2 environment in Travis CI instead of using
the older platform, which has intermittent issues:
1. collect2: fatal error: ld terminated with signal 9 [Killed]
2. ticketlock_autotest sometimes times out

These failures hint at resource availability issues in container
environments. The Graviton2 environment is using VMs and these failures
are not observed in it.

Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Aaron Conole <aconole@redhat.com>