Update README ruby version 2.3->2.4

Author: jdbean

README.md

@@ -83,7 +83,7 @@ Instructions on how to start GitLab and how to run the tests can be found in the
 GitLab is a Ruby on Rails application that runs on the following software:
 
 - Ubuntu/Debian/CentOS/RHEL/OpenSUSE
-- Ruby (MRI) 2.3
+- Ruby (MRI) 2.4
 - Git 2.8.4+
 - Redis 2.8+
 - PostgreSQL (preferred) or MySQL