Update rand crate to 0.5 (#1542)

* Update rand crate to 0.5

* Rustfmt

* Misplaced import

core/src/core/compact_block.rs

@@ -14,7 +14,7 @@
 
 //! Compact Blocks.
 
-use rand::{thread_rng, Rng};
+use rand::{thread_rng, RngCore};
 
 use consensus::VerifySortOrder;
 use core::block::{Block, BlockHeader, Error};