Update to Ruby and Rails supported versions (#2592)

Addresses issue reported in #2590

Update the supported versions of both ruby and rails on the README.md

Author: briri

README.md

@@ -14,8 +14,8 @@ Click here for the latest [releases].(https://github.com/DMPRoadmap/roadmap/rele
 
 #### Pre-requisites
 Roadmap is a Ruby on Rails application and you will need to have:
-* Ruby >= 2.4.4
-* Rails >= 4.2
+* Ruby = 2.4.0 - 2.4.4
+* Rails = 4.2
 * MySQL >= 5.0 OR PostgreSQL
 
 Further detail on how to install Ruby on Rails applications are available from the Ruby on Rails site: http://rubyonrails.org