Use Ruby license in curb.gemspec.erb (#446)

The project is has Ruby license, however, the MIT keeps popping up due to this template. So fix the license in template to fix it once and for all

lib/curb.gemspec.erb

@@ -36,5 +36,5 @@ Gem::Specification.new do |s|
   <% else %>
   s.platform = Gem::Platform::RUBY
   <% end %>
-  s.licenses = ['MIT']
+  s.licenses = ['Ruby']
 end