README: Use SVG Badge (#457)

Better for high DPI screens / scaling and use https for badge fury url to decrease redirects

Signed-off-by: Alex Mayer <amayer5125@gmail.com>

Co-authored-by: James Couball <jcouball@yahoo.com>

README.md

@@ -18,7 +18,7 @@ You can install Ruby/Git like this:
 
 * [![Build Status](https://travis-ci.org/ruby-git/ruby-git.svg?branch=master)](https://travis-ci.org/ruby-git/ruby-git)
 * [![Code Climate](https://codeclimate.com/github/ruby-git/ruby-git.png)](https://codeclimate.com/github/ruby-git/ruby-git)
-* [![Gem Version](https://badge.fury.io/rb/git.png)](http://badge.fury.io/rb/git)
+* [![Gem Version](https://badge.fury.io/rb/git.svg)](https://badge.fury.io/rb/git)
 
 ## Major Objects