WebAssembly support in rand dependency

[kirch7]

I suggest changing the rand dependency version to 0.4.1: https://github.com/rust-lang-nursery/rand/commit/cbd99e5e8822018f028464927dac13beb4007bf6 This would allow building to wasm32-unknown-unknown target.

[zackmdavis]

thanks: 0c41dd17