Update gemspec with correct LICENSE file extension

Author: estolfo

elasticsearch-dsl.gemspec

@@ -22,7 +22,7 @@ Gem::Specification.new do |s|
   s.test_files    = s.files.grep(%r{^(test|spec|features)/})
   s.require_paths = ["lib"]
 
-  s.extra_rdoc_files  = [ "README.md", "LICENSE" ]
+  s.extra_rdoc_files  = [ "README.md", "LICENSE.txt" ]
   s.rdoc_options      = [ "--charset=UTF-8" ]
 
   s.required_ruby_version = '>= 1.9'