Bump required ruby version to v2.4.0

a9137ac introduced a use of Hash#transform_values in
Listen::Record#dir_entries, however that was only added to Ruby in
[v2.4](https://github.com/ruby/ruby/blob/v2_4_0/NEWS#core-classes-updates-outstanding-ones-only-).

This can lead to runtime errors when run on an older version of Ruby,
such as 2.2.7, which the gemspec previously implied was supported.

listen.gemspec

@@ -32,7 +32,7 @@ Gem::Specification.new do |gem| # rubocop:disable Metrics/BlockLength
   gem.executable   = 'listen'
   gem.require_path = 'lib'
 
-  gem.required_ruby_version = '>= 2.2.7' # rubocop:disable Gemspec/RequiredRubyVersion
+  gem.required_ruby_version = '>= 2.4.0' # rubocop:disable Gemspec/RequiredRubyVersion
 
   gem.add_dependency 'rb-fsevent', '~> 0.10', '>= 0.10.3'
   gem.add_dependency 'rb-inotify', '~> 0.9', '>= 0.9.10'