First Pull Request. Passing along unused recurse parameter in get_image_files

Hi all.
Thanks for doing a great work.
I noticed that in fastai/vision/ the function
get_image_files takes a paramterer recurse but never uses it, it should pass it along to get_files
I stumbled upon it when i didnt get any images form a folder with subdirs containing images.
After adding this change it returns the images.


I should perhaps have create a test in tests/, However i am unsure of the practice of file setup/teardown you have.

anyway. This is my first pull request to this repo and i hope to contribute more.