bump rand dependency to 0.5.0-pre.2
This is blocking legacy/trac#23886 (moved), because the rand_core::RngCore
and rand::Rng
traits resolve as being different, because curve25519-dalek
needs to use 0.5.0-pre.2
because it added impl CryptoRng for OsRng
(which we also probably want but I did a workaround in legacy/trac#25508 (moved)).