Improvement: use non-blocking version of `randomBytes`

[midnight-wonderer]

Hello there

I audited the code because the interface is weird; generally, generating a secure random in javascript is a blocking operation, and fernet.js generate IVs without exposing the async interface.

Turns out you are using the blocking version of randomBytes (https://github.com/csquared/fernet.js/blob/2eaa1c/fernet.js#L49).

Consider switching to the promisified version.

const asyncRandomBytes = promisify(randomBytes); // declaration
await asyncRandomBytes(128 / 8); // usage

But probably require a major version bump because the interface will be changed.
What are your takes on this?