Package LICENSE file

The source gemspec file is useless after building the gem file.

Author: nobu

ruby2_keywords.gemspec

@@ -10,8 +10,8 @@ Gem::Specification.new do |s|
   s.authors = ["Nobuyoshi Nakada"]
   s.require_paths = ["lib"]
   s.files = [
+    "LICENSE",
     "README.md",
     "lib/ruby2_keywords.rb",
-    "ruby2_keywords.gemspec",
   ]
 end