Set license code in metadata to MIT

In 5638cb0 the gem was relicensed to
MIT, but it was incomplete.  86c762a at
least fixed the README, but the gemspec itself was still forgotten.

Fixes: 5638cb0

Author: ekohl

secure_headers.gemspec

@@ -14,7 +14,7 @@ Gem::Specification.new do |gem|
     including content-security-policy, x-frame-options,
     strict-transport-security, etc.'
   gem.homepage      = "https://github.com/twitter/secureheaders"
-  gem.license       = "Apache-2.0"
+  gem.license       = "MIT"
   gem.files         = `git ls-files`.split($INPUT_RECORD_SEPARATOR)
   gem.executables   = gem.files.grep(%r{^bin/}).map { |f| File.basename(f) }
   gem.test_files    = gem.files.grep(%r{^(test|spec|features)/})