Require ruby-2.4+ in gemspec

Since we don't test older rubies.

pg-ldap-sync.gemspec

@@ -19,6 +19,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
   spec.rdoc_options = %w[--main README.md --charset=UTF-8]
+  spec.required_ruby_version = ">= 2.4"
 
   spec.add_runtime_dependency "net-ldap", "~> 0.16"
   spec.add_runtime_dependency "kwalify", "~> 0.7"