Update rand requirement from 0.4.2 to 0.5.5

Updates the requirements on [rand](https://github.com/rust-lang-nursery/rand) to permit the latest version.
- [Release notes](https://github.com/rust-lang-nursery/rand/releases)
- [Changelog](https://github.com/rust-lang-nursery/rand/blob/master/CHANGELOG.md)
- [Commits](https://github.com/rust-lang-nursery/rand/commits/0.5.5)

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

test/Cargo.toml

@@ -4,7 +4,7 @@ name = "wee_alloc_test"
 version = "0.1.0"
 
 [dependencies]
-rand = "0.4.2"
+rand = "0.5.5"
 quickcheck = "0.6.0"
 histo = "0.1.0"
 cfg-if = "0.1.2"