Prepare release 2.5.0

Author: Anthony ROUX

README.md

@@ -11,7 +11,7 @@ Amadeus provides a rich set of APIs for the travel industry. For more details, c
 
 ## Installation
 
-This gem requires Ruby 2.4+. You can install it directly or via bundler.
+This gem requires Ruby 2.5+. You can install it directly or via bundler.
 #### Command line
 ```bash
 gem install amadeus

lib/amadeus/version.rb

@@ -2,5 +2,5 @@
 
 module Amadeus
   # The current client version
-  VERSION = '5.1.0'
+  VERSION = '5.2.0'
 end