rapidjson

ci: upgrade distro to xenial and add arm64 test cases (#1662)

Start from xenial, Travis supports multiple CPU architectures.
To bump to this version allows expand test coverage for more
architectures.
See: https://docs.travis-ci.com/user/reference/overview/#virtualisation-environment-vs-operating-system
Add arm64 test cases to matrix.

Change-Id: If61e2d38223dad70b542d6ec0afcf4a433c9debf
Signed-off-by: Jun He <jun.he@arm.com>

Co-authored-by: Jun He <jun.he@arm.com>