Set minimum Ruby version to 2.6

Author: Mattia Roccoberton

activeadmin_active_resource.gemspec

@@ -21,6 +21,8 @@ Gem::Specification.new do |spec|
   spec.files         = Dir['{lib}/**/*', 'LICENSE.txt', 'Rakefile', 'README.md']
   spec.require_paths = ['lib']
 
+  spec.required_ruby_version = '>= 2.6.0'
+
   spec.add_runtime_dependency 'activeadmin', '~> 2.0'
   spec.add_runtime_dependency 'activeresource', '>= 5.1'