README: Use SVG build badge from GitHub Actions (igrigorik#360)

...since this can report the status.

[ci skip]

Author: olleolleolle

README.md

@@ -1,7 +1,7 @@
 # EM-HTTP-Request 
 
 [![Gem Version](https://badge.fury.io/rb/em-http-request.svg)](http://rubygems.org/gems/em-http-request)
-[![GitHub Workflow Status](https://img.shields.io/github/actions/workflow/status/igrigorik/em-http-request/ci.yml)](https://github.com/igrigorik/em-http-request/actions/workflows/ci.yml)
+[![CI](https://github.com/igrigorik/em-http-request/actions/workflows/ci.yml/badge.svg)](https://github.com/igrigorik/em-http-request/actions/workflows/ci.yml)
 
 Async (EventMachine) HTTP client, with support for: