Add the pstore gem for Ruby 4.0 and YAML::Store (closes #91).

* Ruby 4.0 removed the `pstore` gem from the standard set of default gems.
  As a workaround it must be added to the `Gemfile`.

Author: postmodern

Gemfile

@@ -4,6 +4,7 @@ source 'https://rubygems.org'
 
 gemspec
 
+gem 'pstore' if RUBY_VERSION >= '4.0.0'
 gem 'jruby-openssl',	'~> 0.7', platform: :jruby
 
 # gem 'command_kit', '0.5.0', github: 'postmodern/command_kit',