Update the license for the default gems to dual licenses

Author: hsbt

weakref.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |spec|
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = "https://github.com/ruby/weakref"
-  spec.license       = "BSD-2-Clause"
+  spec.licenses      = ["Ruby", "BSD-2-Clause"]
 
   # Specify which files should be added to the gem when it is released.
   # The `git ls-files -z` loads the files in the RubyGem that have been added into git.