Replace travis with GitHub Actions

[dcroote]

As mentioned in #11 (comment), here is the GitHub Actions build replacement for travis CI. It uses a matrix strategy with 3 targets and leverages the built-in cross functionality of the actions-rs/cargo@v1 Action.

Here is the successful Actions run on my fork.

[rahul-thakoor]

thanks, @dcroote :)

what's the easiest way to publish automatically to crates.io?

maybe we publish only prs with semantic version tags?

[dcroote]

Hi @rahul-thakoor - sure thing!

I added a job to the workflow that depends on the build job and publishes only push commits with a tag to crates.io. Note you'll need to go to this repo's Settings, then Secrets, and add a new repository secret named CRATES_IO_TOKEN for this to work.

[rahul-thakoor]

thanks !