Add license to gemspec

[ghost]

• Show gem license on rubygems.org

Our open source developers this week have submitted a pull request to rubygems.org to display a gem's license on the gem version's page.

Make sure you add a license to your gemspecs!

Gem::Specification.new do |s|
  s.license = "MIT"
  #....
end

or...

Gem::Specification.new do |s|
  s.licenses = ["MIT", "BSD"]
  #....
end

Pull request: rubygems/rubygems.org#458