chore: update haml dependency version

This updates to the haml dependency relative to the current version.
haml 3 has a cross scripting security issue with ruby. To avoid glitter
installing an insecure of version haml where this is the only gem with a
haml dependency, the version is set to ~>5. This also removes the
deprecated `rubyforge_project` property in the gemspec.

glitter.gemspec

@@ -13,10 +13,9 @@ Gem::Specification.new do |s|
   s.description = %q{Glitter makes it easy to publish software updates via the Sparkle framework by using S3 buckets.}
   s.licenses    = ['MIT']
 
-  s.rubyforge_project = "glitter"
   s.required_ruby_version = ">= 2.0.0"
   s.add_dependency "s3", "~> 0.3"
-  s.add_dependency "haml", "~> 3.0"
+  s.add_dependency "haml", "~> 5.0"
   s.add_dependency "thor", "~> 1.0"
 
   s.files         = `git ls-files`.split("\n")