use `alias_method_chain` rather than overriding `render` directly

I noticed this wasn't playing nicely with [wicked_pdf](https://github.com/mileszs/wicked_pdf), which uses `alias_method_chain` to give different behaviour to the `render` method. FlashRender wasn't getting called as as result. Adding `alias_method_chain` to this gem fixes the issue (swapping the order of the gems in my Gemfile also worked, but this seems like a better way).