Fix dependency with http:

* Since version 5.0.0 http provides the method mkcalendar to make requests. Since the gem makes use of that method, this version of http is necessary;
Ref: httprb/http#660
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calendav.gemspec

@@ -24,7 +24,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_runtime_dependency "http"
+  spec.add_runtime_dependency "http", "~> 5.0"
   spec.add_runtime_dependency "icalendar"
   spec.add_runtime_dependency "nokogiri"