Unable to retrieve keys for m/888' derivations paths

[kyranjamie]

Related #102

Stacks authentication uses a the derivation path m/888'/0'/0'/<account> to generate "identify" keys.

The Stacks Ledger app doesn't work when passing a m/888' leading derivation path, returning the error Data is invalid. This is preventing us getting the public keys for this path, and may also impact our ability to use #102