Specify ruby version (#45)

* Specify ruby version

* better

Author: Thomas Hareau

safe-pg-migrations.gemspec

@@ -20,7 +20,7 @@ Gem::Specification.new do |s|
   s.license = 'MIT'
 
   s.platform              = Gem::Platform::RUBY
-  s.required_ruby_version = '>= 2.5'
+  s.required_ruby_version = '>= 2.5', '< 4'
 
   s.add_dependency 'activerecord', '>= 5.2'
   s.add_dependency 'activesupport', '>= 5.2'