Rake is now required at runtime

mimemagic.gemspec

@@ -20,9 +20,9 @@ Gem::Specification.new do |s|
   s.license = 'MIT'
 
   s.add_dependency('nokogiri', '~> 1.11.2')
+  s.add_dependency('rake')
 
   s.add_development_dependency('minitest', '~> 5.14')
-  s.add_development_dependency('rake', '~> 13.0')
 
   if s.respond_to?(:metadata)
     s.metadata['changelog_uri'] = "https://github.com/mimemagicrb/mimemagic/blob/master/CHANGELOG.md"