Allow Haml 5.0.0

Haml 5.0.0.beta.2 has been out for a while and it packs some 
interesting fixes that can't be tested until haml-rails allows for it

Author: olivierlacan

haml-rails.gemspec

@@ -15,7 +15,7 @@ Gem::Specification.new do |s|
   s.rubyforge_project         = "haml-rails"
   s.required_rubygems_version = ">= 1.3.6"
 
-  s.add_dependency "haml",          [">= 4.0.6", "< 5.0"]
+  s.add_dependency "haml",          [">= 4.0.6", "< 6.0"]
   s.add_dependency "activesupport", [">= 4.0.1"]
   s.add_dependency "actionpack",    [">= 4.0.1"]
   s.add_dependency "railties",      [">= 4.0.1"]