Migrate CI to Github Actions

[felixbr]

This is a first attempt. I copied most of it from typelevel/cats.

[travisbrown]

Have you looked at sbt-github-actions? I used it recently for Dhall for Java and @jatcwang used it for several Circe projects and it seems pretty convenient.

[travisbrown]

(I'd also be happy to have us merge this as-is.)

[felixbr]

I actually looked at sbt-github-actions first but since cats, doobie and other projects seemed to not use it, I just followed their example. I didn't realize that the plugin is used to generate the yaml files but it makes sense in retrospect.

I'll clean up the PR and we can merge this and I'll take a look at the sbt-plugin again and possibly open another one. For now I just wanted to make sure to migrate before we run into trouble with travis-ci (I couldn't find a clear deadline, so I'd assume rather sooner than later). Right now it aims to be a 1-to-1 migration of the travis-ci build (minus sbt-travisci).

I'm not too familiar with codecov but I assume the bot will only post in the PR if something has changed in the code.

[codecov-io]

Codecov Report

Merging #288 (26acf89) into master (f5f05e9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #288   +/-   ##
=======================================
  Coverage   91.35%   91.35%           
=======================================
  Files           9        9           
  Lines         162      162           
  Branches        3        3           
=======================================
  Hits          148      148           
  Misses         14       14           

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