openssl.gemspec: add BSD-2-Clause to the list of licenses

ruby/openssl is licensed under the terms of either the Ruby License or
the 2-Clause BSD License.

The git repository and built .gem files always contained the license
text for both license, but the metadata in the gemspec only specified
the Ruby License. Let's include both.

Author: rhenium

openssl.gemspec

@@ -6,7 +6,7 @@ Gem::Specification.new do |spec|
   spec.summary       = %q{SSL/TLS and general-purpose cryptography for Ruby}
   spec.description   = %q{OpenSSL for Ruby provides access to SSL/TLS and general-purpose cryptography based on the OpenSSL library.}
   spec.homepage      = "https://github.com/ruby/openssl"
-  spec.license       = "Ruby"
+  spec.licenses      = ["Ruby", "BSD-2-Clause"]
 
   if Gem::Platform === spec.platform and spec.platform =~ 'java' or RUBY_ENGINE == 'jruby'
     spec.platform    = "java"