Update ci status badges

Remove obsolete travis and appveyor build status badges, and add
one for github actions build.

Author: rillian

README.md

@@ -1,8 +1,7 @@
 # Opusfile
 
 [![GitLab Pipeline Status](https://gitlab.xiph.org/xiph/opusfile/badges/master/pipeline.svg)](https://gitlab.xiph.org/xiph/opusfile/commits/master)
-[![Travis Build Status](https://travis-ci.org/xiph/opusfile.svg?branch=master)](https://travis-ci.org/xiph/opusfile)
-[![AppVeyor Build Status](https://ci.appveyor.com/api/projects/status/github/xiph/opusfile?branch=master&svg=true)](https://ci.appveyor.com/project/rillian/opusfile)
+[![GitHub CI](https://github.com/xiph/opusfile/actions/workflows/build.yml/badge.svg)](https://github.com/xiph/opusfile/actions/workflows/build.yml)
 
 The opusfile and opusurl libraries provide a high-level API for
 decoding and seeking within .opus files on disk or over http(s).