Add LICENSE to .gemspec

This will include the LICENSE file in the distributed .gem.

Author: Josef Stribny

sprockets-rails.gemspec

@@ -6,7 +6,7 @@ Gem::Specification.new do |s|
   s.summary  = "Sprockets Rails integration"
   s.license  = "MIT"
 
-  s.files = Dir["README.md", "lib/**/*.rb"]
+  s.files = Dir["README.md", "lib/**/*.rb", "LICENSE"]
 
   s.add_dependency "sprockets", "~> 2.8"
   s.add_dependency "actionpack", ">= 3.0"