TaskRng -> ThreadRng, task_rng -> thread_rng

Fix for current rust nightly, I've renamed:
 * `TaskRng` to `ThreadRng`
 * `task_rng` to `thread_rng`

See: https://github.com/rust-lang/rust/commit/1e89bbcb67020892bc0af5af218c35f0fd453fa4