Update rand requirement from 0.7.2 to 0.8.0

Updates the requirements on [rand](https://github.com/rust-random/rand) to permit the latest version.
- [Release notes](https://github.com/rust-random/rand/releases)
- [Changelog](https://github.com/rust-random/rand/blob/master/CHANGELOG.md)
- [Commits](rust-random/rand@0.7.2...0.8.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

foundationdb-bench/Cargo.toml

@@ -24,6 +24,6 @@ env_logger = "0.7.1"
 foundationdb = { version = "*", path = "../foundationdb" }
 futures = "0.3.0"
 log = "0.4.8"
-rand = "0.7.2"
+rand = "0.8.0"
 stopwatch = "0.0.7"
 structopt = "0.3.3"

foundationdb/Cargo.toml

@@ -45,7 +45,7 @@ foundationdb-gen = { version = "0.5.1", path = "../foundationdb-gen", default-fe
 foundationdb-sys = { version = "0.5.1", path = "../foundationdb-sys", default-features = false }
 futures = "0.3.1"
 memchr = "2.2.1"
-rand = { version = "0.7.2", features = ["default", "small_rng"] }
+rand = { version = "0.8.0", features = ["default", "small_rng"] }
 static_assertions = "1.1.0"
 uuid = { version = "0.8.1", optional = true }
 num-bigint = { version = "0.3.0", optional = true }