Update Readme

In Gemfile, `~> 3.3.0` will look for version `3.3.x`, so if user follow the readme will not get the latest version.

Author: zillou

Readme.md

@@ -23,7 +23,7 @@ Due to an error displaying the plus icon in a recent version of this gem, the re
 1 - Add to your Gemfile:
 
 ```ruby
-gem 'jquery-datatables-rails', '~> 3.3.0'
+gem 'jquery-datatables-rails', '~> 3.4.0'
 ```
 
 2 - Install the gem: