Add activesupport as a dependency and require AS/dependencies

The gem won't load without it.

lib/secure_headers/railtie.rb

@@ -1,3 +1,5 @@
+require 'active_support/dependencies'
+
 # rails 3.1+
 if defined?(Rails::Railtie)
   module SecureHeaders

secure-headers.gemspec

@@ -19,5 +19,6 @@ Gem::Specification.new do |gem|
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})
   gem.require_paths = ["lib"]
   gem.add_dependency "brwsr", ">= 1.1.1"
+  gem.add_dependency "activesupport", ">= 2.0"
   gem.add_development_dependency "rake"
 end