Bump dependency on 'rand'
Bump dependency on 'rand'

rand 0.3 depends on rand 0.4 anyway, so the dependency
graph can only get simpler, if it changes at all.

The only potentially-breaking changes in rand seem
to be centered around 'thread_rng' and 'sample'.
Neither are used directly.

Razaekel/noise-rs,bjz/noise-rs