message: make rand's version static in the chain-impl-mockchain
diff: @@ -16,7 +16,7 @@ bincode = "^1.0.1"
  quickcheck = { version = "0.8" }
  chain-core = { path = "../chain-core" }
  chain-addr = { path = "../chain-addr" }
  -rand = "*"
  +rand = "0.6"
  [dependencies.cardano]
  path = "../cardano"
  