Follow conventions to save cached versions in appropriate location

[Siilwyn]

No idea why you would cache versions 'globally' like this in the first place but...
Instead of placing the cached files in the home directory it should be placed in the appropriate directory per OS. I know there is an env variable but following convention should be a good default.

What is the current behavior?

Saving everything in ~/.gitbook.

What is the expected behavior?

Saving everything in the appropriate directory per OS, e.g. for Linux: ~/.cache/gitbook.

See same issues in other tools: greenkeeper, atom, js-v8flags, electron-download.

[Siilwyn]

RIP gitbook in it's local form, for anybody stumbling on this I can recommend mdBook as an alternative.