Update gem badge to .svg

Author: crispgm

README.md

@@ -2,7 +2,7 @@
 
 Automatically generate post archives by dates, tags, and categories.
 
-[![Gem Version](https://badge.fury.io/rb/jekyll-archives.png)](http://badge.fury.io/rb/jekyll-archives)
+[![Gem Version](https://badge.fury.io/rb/jekyll-archives.svg)](http://badge.fury.io/rb/jekyll-archives)
 [![Build Status](https://travis-ci.org/jekyll/jekyll-archives.svg?branch=master)](https://travis-ci.org/jekyll/jekyll-archives)
 
 ## Getting started