Update README to 2.4.0

Author: michael-misshore

README.md

@@ -27,7 +27,7 @@ However, more packages may need to be installed depending on your OS distributio
 After reviewing the dependency requirements, add `critical-path-css-rails` to your Gemfile:
 
 ```
-gem 'critical-path-css-rails', '~> 2.3.0'
+gem 'critical-path-css-rails', '~> 2.4.0'
 ```
 
 Download and install by running: