README.md in rakuten_web_service-1.7.0 vs README.md in rakuten_web_service-1.8.0

- old
+ new

@@ -28,10 +28,10 @@
 * [Contributing](#contributing)
 
 ## Prerequisite
 
-* Ruby 2.1.0 or later
+* Ruby 2.2 or later
 
 ## Installation
 
 Add this line to your application's Gemfile: