This commit introduces a Github Actions workflow in the form of the file
`.github/workflows/build.yml`, replacing Travis CI and its `.travis.yml`
file.
The new workflow does a little bit more than Travis CI: it uploads an
artifact after a successful build. Due to a known issue with Github's
package UI, the MobArena.jar file is dynamically zipped on download, so
we get MobArena.jar.zip containing just MobArena.jar. It's redundant,
but there doesn't seem to be a simple way around it at the time of this
commit, so we'll leave it be.
For now, the workflow runs on every push to every branch. I'm probably
going to regret that, but we'll leave that as it is for now as well.
