Improve rubocop settings

[gssbzn]

• Add .rubocop_todo.yml file and reference it on settings file
• Apply recent rubocop autocorrections to code base

[tarcieri]

I'd prefer to leave nothing to .rubocop_todo.yml, otherwise this seems good to me.

[gssbzn]

@tarcieri What do you mean by "leave leave nothing to .rubocop_todo.yml"
Leave the file empty? or remove any reference to it?
Some of those "todo" warnings were already on the settings file.
Regenerate the file with each fix, which IMO will be easier than having to check the rubocop.yml when they are fixed.
Let me know so I can fix the PR

[tarcieri]

@gssbzn I mean either fix the problems or add appropriate overrides to .rubocop.yml to use in perpetuity

[gssbzn]

@tarcieri I removed the reference to the todo file
Later I can do another PR with the code style fixes needed to remove this warnings while I get more familiar with the code base

Let me know if this is fine or if there's another thing I should look at

[tarcieri]

I guess you're declaring bankruptcy on older versions of libsodium that don't implement scrypt here?

[gssbzn]

This was the harder part to try to fix
I was checking the ffi gem, and the Sodium module and my conclusion was that the rescue was there for the sodium_function calls.
But at the same time I checked for other classes that use the sodium_function and they don't rescue in case a definition with fii fails.
Is this rescue only for scrypt case? Should the scrypt function only be exposed through RbNaCl::PasswordHash? If that's the case maybe only a require of this class in this module and handle the rescue there.

[gssbzn]

Or I can update the README to reflect the minimum libsodium

[tarcieri]

Yeah, looks like scrypt was added prior to 1.0. I think it's fine to note in the README that the minimum version is 1.0.0.

[gssbzn]

Done

[tarcieri]

This looks fine with one noted caveat