Update the license for the default gems to dual licenses

Author: hsbt

benchmark.gemspec

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{a performance benchmarking library}
   spec.description   = spec.summary
   spec.homepage      = "https://github.com/ruby/benchmark"
-  spec.license       = "BSD-2-Clause"
+  spec.licenses       = ["Ruby", "BSD-2-Clause"]
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage