Added BSD-2-Clause to the licenses of the gemspec

Author: nobu

ruby2_keywords.gemspec

@@ -5,7 +5,7 @@ Gem::Specification.new do |s|
   s.version = version
   s.summary = "Shim library for Module#ruby2_keywords"
   s.homepage = "https://github.com/ruby/ruby2_keywords"
-  s.licenses = ["Ruby"]
+  s.licenses = ["Ruby", "BSD-2-Clause"]
   s.authors = ["Nobuyoshi Nakada"]
   s.require_paths = ["lib"]
   s.files = [