Upgrade from gmake to gmake2 (requires premake 5.0 alpha 12)
Change instructions to indicate VS2019 instead of 2015
Delete .travis.yml configuration
Add a GitHub Workflow that checks out & runs tests
Update README badge
ci.yml has a few improvements from the old Travis flow:
Targets Windows and MacOS in addition to Linux
Fetches libsodium from apt for faster builds
Builds mbedtls3 from source (since only mbedtls2 is available in
Ubuntu 20.04 LTS' repositories), but caches the output so that we can
quickly retrieve it from GH's cache when we build
ci.yml has a few improvements from the old Travis flow:
Successful run: https://github.com/jorgenpt/yojimbo/actions/runs/1427137706