travis-ci.org builds have ceased, so this patch adds a GitHub Actions
workflow to build and test the extension, as well as a Dependabot
configuration file to keep the referenced actions up to date.
I've opted to slightly simplify the workflow compared to the Travis
build script. For one, I have not included libmemcached protocol
support, as most distros do not supply a libmemcached compiled with
server protocol support, and previous discussion suggests its usefulness
in the context of this extension may be limited.[1] I've also opted to
remove multiversion libmemcached testing in favor of using the system
libmemcached, as the referenced other versions seem to be old enough for
this to hopefully not be a problem. These could conceivably be
reintroduced if needed.
---
[1] https://github.com/php-memcached-dev/php-memcached/issues/418#issuecomment-449587972
