Add .gemspec files to 2-3-stable to help Bundler

Would you please consider including gemspec files in the 2-3-stable branch? This would help folks with ancient 2.3 apps by allowing them to use Bundler without requiring new official gem releases. 

Or, would it be better for folks to maintain a separate 2.3 fork?

This is related to [pull request 8948](https://github.com/rails/rails/pull/8948)