Use 'terraform fmt -check' instead of manually checking diff

[choznerol]

First, thanks for creating this repo. I've learned a lot from it!

Because we use this repo as a starting template for our own project, I think it might be helpful to upstream this small tweak back to this repo.

-check - Check if the input is formatted. Exit status will be 0 if all input is properly formatted and non-zero otherwise. -recursive - Also process files in subdirectories. By default, only the given directory (or current directory) is processed. https://www.terraform.io/cli/commands/fmt#usage