- Use raw.githubusercontent.com to download cask source rather than
formulae.brew.sh. This allows us to remove these files
- output the tap's current `HEAD` for both formulae and cask JSON
- use this `HEAD` for the cask-source API to get the exact file on
raw.githubusercontent.com rather than just whatever is newest (which
is what the previous API did)
- set the `Tap` correctly when creating a `Cask` from the API
- if the `formula.json` file exists: print its modified time include
`brew config`
- memoize `tap.git_head` as we'll be calling it a lot in the same
process with the same value
