Relax Haml dependency to allow for newer versions

Haml 5.0.0 beta 2 has been out since January and since haml-rails depends on
html2haml, it's impossible to test this version out while using haml-rails.

html2haml.gemspec

@@ -21,7 +21,7 @@ Gem::Specification.new do |gem|
   gem.add_dependency 'nokogiri', '>= 1.6.0'
   gem.add_dependency 'erubis', '~> 2.7.0'
   gem.add_dependency 'ruby_parser', '~> 3.5'
-  gem.add_dependency 'haml', '~> 4.0'
+  gem.add_dependency 'haml', ['>= 4.0', '< 6']
   gem.add_development_dependency 'simplecov', '~> 0.7.1'
   gem.add_development_dependency 'minitest', '>= 4.4.0'
   gem.add_development_dependency 'rake'