Upgrade rand to 0.3.15
Created by: aesedepece
After https://github.com/rust-lang-nursery/rand/commit/f2267c1d03b22349d7dedc1d823ebd115f820341, there is no 0.3.14
release for https://github.com/rust-lang-nursery/rand .
This commit fixes compilation of redox by upgrading rand's version to 0.3.15
.