Change the template hash function to SHA-256.

[SHA-1](https://en.wikipedia.org/wiki/SHA1) is slowly losing its footing, and [SHA-256](https://en.wikipedia.org/wiki/SHA256) is a better choice going forward. (SHA3/Keccak is still not finalized.)

SHA-256 is already supported by `homebrew-cask` in exactly the same way as SHA-1. In fact, my submitted casks have been using SHA-256.

This pull request doesn't break any functionality (casks with SHA-1 hashes will continue to work), but the template from `brew cask create` will now have a field for SHA-256 by default.