Merge pull request thoughtbot#309 from caarlos0/license-missing-gemspec

adding MIT license to gemspec - fixes thoughtbot#302

bourbon.gemspec

@@ -8,6 +8,7 @@ Gem::Specification.new do |s|
   s.platform    = Gem::Platform::RUBY
   s.authors     = ["Phil LaPier", "Chad Mazzola", "Matt Jankowski", "Nick Quaranto", "Jeremy Raines", "Mike Burns", "Andres Mejia", "Travis Haynes", "Chris Lloyd", "Gabe Berke-Williams", "J. Edward Dewyea", "Reda Lemeden"]
   s.email       = ["[email protected]"]
+  s.license     = 'MIT'
   s.homepage    = "https://github.com/thoughtbot/bourbon"
   s.summary     = "Bourbon Sass Mixins using SCSS syntax."
   s.description = <<-DESC