Add version and checksum info to BasicTeX

[cubranic]

No description provided.

[nanoxd]

The url points to a non-versioned cask. Adding this in would require a shasum update for every change at the URL. For now version 'latest' is still preferred until #4678 is in place.

[cubranic]

Isn't that how even versioned casks work when a new version of the app is released? Update the "version" field and update the "sha256". Yes, there is a brief window when a new version is released when shasum will differ and installing will fail, but for BasicTeX this only happens once a year, on a predictable schedule. As a user, I'd rather know that my TeX installation needs to be upgraded, as I'll know when I see a new commit of the caskfile. (And yes, I am happy to keep the formula up to date.)

Anyways, I found a versioned URL on the mirror and have updated the formula to use it.