Allow newer versions of Cucumber

This allows us to test / release new versions of Cucumber without
first having to come here to make a new release of Arubua.

I think the only reason to cap the max version would be if we knew there
was a backwards-compatibility problem with a release of Cucumber, but
there isn't, so let's remove it?

aruba.gemspec

@@ -23,7 +23,7 @@ Gem::Specification.new do |spec|
 
   spec.add_runtime_dependency "childprocess", [">= 2.0", "< 5.0"]
   spec.add_runtime_dependency "contracts", "~> 0.16.0"
-  spec.add_runtime_dependency "cucumber", [">= 2.4", "< 6.0"]
+  spec.add_runtime_dependency "cucumber", ">= 2.4"
   spec.add_runtime_dependency "rspec-expectations", "~> 3.4"
   spec.add_runtime_dependency "thor", "~> 1.0"