README: fix badge, switch to GitHub actions

Author: tarleb

README.md

@@ -1,10 +1,9 @@
 # Lua Filters
 
-[![Travis build status][Travis badge]](https://travis-ci.org/pandoc/lua-filters)
-[![Circle CI build status][Circle CI badge]](https://circleci.com/gh/pandoc/lua-filters)
+[![Build status][GitHub Actions badge]][GitHub Actions]
 
-[Travis badge]: https://img.shields.io/travis/pandoc/lua-filters.svg?logo=travis
-[Circle CI badge]: https://img.shields.io/circleci/build/gh/pandoc/lua-filters?logo=circleci
+[GitHub Actions badge]: https://img.shields.io/github/workflow/status/pandoc/lua-filters/CI?logo=github
+[GitHub Actions]: https://github.com/pandoc/lua-filters/actions
 
 This repository collects Lua filters for pandoc.