updated README to include Ruby 2.1

Former-commit-id: 47e0d6b1

README.md

@@ -67,7 +67,7 @@ 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
-- Ruby (MRI) 2.2
+- Ruby (MRI) 2.1 or 2.2
 - Git 1.7.10+
 - Redis 2.8+
 - MySQL or PostgreSQL