authors may not be empty

Author: nobu

ruby2_keywords.gemspec

@@ -6,6 +6,7 @@ Gem::Specification.new do |s|
   s.summary = "Shim library for Module#ruby2_keywords"
   s.homepage = "https://www.ruby-rang.org/"
   s.licenses = ["Ruby"]
+  s.authors = ["Nobuyoshi Nakada"]
   s.require_paths = ["lib"]
   s.files = [
     "lib/ruby2_keywords.rb",