Builds and tests on arm64 architecture

[Martin Grigorov]

Hello Memcached team,

I've just opened https://github.com/memcached/memcached/pull/593 with a small change in .travis.yml to enable build and test at both amd64 (as now) and arm64.

ARM64 is used more and more for servers too and it would be good if Memcached is tested regularly.
I've ran the build and tests both on one of our arm64 machines and at TravisCI and at the moment both are fine!

I hope this change would be accepted!

Regards,

Martin