Update rand to ^0.4.2
Update rand to ^0.4.2

RustySecrets makes minimal use of the rand library. It only initializes
the `ChaChaRng` with a seed, and `OsRng` in the standard way, and then calls
their `fill_bytes` methods, provided by the same Trait, and whose function
signature has not changed.  I have confirmed by looking at the code changes,
that there have been no changes to the relevant interfaces this library uses.

SpinResearch/RustySecrets