Upgrade dependencies

[Stigjb]

The dependencies are quite old, and I couldn't compile the crate in my project which uses a more recent version of rand.

I've made the num dependency a greater range of versions, as long as they are below v1.

I've raised the minimum version of rand to 0.5, which is when the Rand trait was deprecated, and changed the implementation of Rand for Letter to conform to the newer API.

I've successfully compiled the crate with recent versions of rand on my PR branch.

[Stigjb]

It turns out that everything from num used by this crate is in num-traits, so that's a few dependencies saved.

I understeed that you wanted the dependencies to be the latest versions, which will lose compatibility with older versions of rand and num - correct me if I'm wrong