Re-add Gem version after accidental removal.

README.markdown

@@ -1,11 +1,13 @@
 # [Jekyll](http://jekyllrb.com/)
 
+[![Gem Version](https://img.shields.io/gem/v/jekyll.svg)][ruby-gems]
 [![Build Status](https://travis-ci.org/jekyll/jekyll.svg?branch=master)][travis]
 [![Test Coverage](https://codeclimate.com/github/jekyll/jekyll/badges/coverage.svg)][coverage]
 [![Code Climate](https://codeclimate.com/github/jekyll/jekyll/badges/gpa.svg)][codeclimate]
 [![Dependency Status](https://gemnasium.com/jekyll/jekyll.svg)][gemnasium]
 [![Security](https://hakiri.io/github/jekyll/jekyll/master.svg)][hakiri]
 
+[ruby-gems]: https://rubygems.org/gems/jekyll
 [gemnasium]: https://gemnasium.com/jekyll/jekyll
 [codeclimate]: https://codeclimate.com/github/jekyll/jekyll
 [coverage]: https://codeclimate.com/github/jekyll/jekyll/coverage