[-] Readme - Add the gem version badge

Author: arnaudbesnier

README.md

@@ -1,4 +1,6 @@
-# Forest Admin in Rails [![Build Status](https://travis-ci.org/ForestAdmin/forest-rails.svg?branch=master)](https://travis-ci.org/ForestAdmin/forest-rails)
+# Forest Admin in Rails
+[![Gem](https://badge.fury.io/rb/forest_liana.svg)](https://badge.fury.io/rb/forest_liana)
+[![CI status](https://travis-ci.org/ForestAdmin/forest-rails.svg?branch=master)](https://travis-ci.org/ForestAdmin/forest-rails)
 
 Forest Admin provides an off-the-shelf administration panel based on a highly-extensible API plugged into your application.