Rename LICENSE.txt to COPYING

This is for consistency with ruby/ruby.

Author: rhenium

LICENSE.txt renamed to COPYING

@@ -13,7 +13,7 @@ Gem::Specification.new do |spec|
     spec.files       = []
     spec.add_runtime_dependency('jruby-openssl', '~> 0.14')
   else
-    spec.files         = Dir["lib/**/*.rb", "ext/**/*.{c,h,rb}", "*.md", "BSDL", "LICENSE.txt"]
+    spec.files         = Dir["lib/**/*.rb", "ext/**/*.{c,h,rb}", "*.md", "BSDL", "COPYING"]
     spec.require_paths = ["lib"]
     spec.extensions    = ["ext/openssl/extconf.rb"]
   end