Add Rails license to lib/activesupport/current_attributes.rb

[rafaelfranca]

That code is copied from the rails code base, so we need to make sure we respect the MIT license and keep the original copyright notice and license on the top of that file.

https://github.com/rails/rails/blob/master/activesupport/MIT-LICENSE

[rafaelfranca]

Also it would be good if we only load that file in versions of rails that don't have that feature already.