Follow conventions to save cached versions 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.

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.

[AaronO]

@Siilwyn It's more of a "content directory" than a "cache directory".

[Siilwyn]

It has different content per version?
Btw. I moved it to the CLI repository: GitbookIO/gitbook-cli#62