Update rand requirement from 0.7 to 0.8

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@0.7.0...0.8.0)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

mla/Cargo.toml

@@ -12,7 +12,7 @@ readme = "../README.md"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-rand = "0.7"
+rand = "0.8"
 rand_chacha = "0.2"
 brotli = "3.3"
 aes-ctr = "0.5"

mlar/Cargo.toml

@@ -17,7 +17,7 @@ clap = "2"
 glob = "0.3"
 mla = { path = "../mla", version = "1" }
 curve25519-parser = { path = "../curve25519-parser", version = "0.2" }
-rand = "0.7"
+rand = "0.8"
 x25519-dalek = "1"
 humansize = "1"
 hex = "0.4"