Fix Ruby version requirement (#432)

This library depends on AR/AS 6.1, which require Ruby 2.5

Close #431

acts_as_list.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |s|
   s.summary               = "A gem adding sorting, reordering capabilities to an active_record model, allowing it to act as a list"
   s.description           = 'This "acts_as" extension provides the capabilities for sorting and reordering a number of objects in a list. The class that has this specified needs to have a "position" column defined as an integer on the mapped database table.'
   s.license               = "MIT"
-  s.required_ruby_version = ">= 2.4.7"
+  s.required_ruby_version = ">= 2.5"
 
   if s.respond_to?(:metadata)
     s.metadata['changelog_uri']   = 'https://github.com/brendon/acts_as_list/blob/master/CHANGELOG.md'