adding license information

aasm.gemspec

@@ -11,6 +11,7 @@ Gem::Specification.new do |s|
   s.summary     = %q{State machine mixin for Ruby objects}
   s.description = %q{AASM is a continuation of the acts as state machine rails plugin, built for plain Ruby objects.}
   s.date        = Time.now
+  s.licenses    = ["MIT"]
 
   s.add_dependency             'activerecord'
   s.add_development_dependency 'rake'