Switch from Travis CI to GitHub Actions

Switch from Travis CI to GitHub Actions

Travis has imposed quotas on builds that require us to contact them to
obtain OSS credits. The CI tool is irrelevant so long as builds do
happen, so switch to GitHub Actions for now. This is a little more
complicated that the switch for related projects like git-pw, given the
need for a database service. We may wish to investigate reusing some of
our own Docker files in the future but for now, this should do the
trick.

Signed-off-by: Stephen Finucane <stephen@that.guru>