Remove Travis CI and add github action badge

Author: nickspoons

.travis.yml

@@ -2,7 +2,7 @@
   <img alt="OmniSharp" src="https://raw.github.com/OmniSharp/omnisharp-vim/gh-pages/omnisharp-logo-light.png">
 </h1>
 
-[![Travis status](https://api.travis-ci.org/OmniSharp/omnisharp-vim.svg)](https://travis-ci.org/OmniSharp/omnisharp-vim)
+![Test status](https://github.com/OmniSharp/omnisharp-vim/actions/workflows/vader.yml/badge.svg)
 [![AppVeyor status](https://ci.appveyor.com/api/projects/status/github/OmniSharp/omnisharp-vim?svg=true)](https://ci.appveyor.com/project/nickspoons/omnisharp-vim)
 
 OmniSharp-vim is a plugin for Vim to provide IDE like abilities for C#.