Merge pull request #35 from coreos/dependabot/cargo/nix-gte-0.18-and-lt-0.22

Update nix requirement from >= 0.18, < 0.21 to >= 0.18, < 0.22

Author: Luca Bruno

Cargo.toml

@@ -16,7 +16,7 @@ documentation = "http://docs.rs/openat-ext"
 [dependencies]
 openat = "0.1.15"
 libc = "^0.2.94"
-nix = ">= 0.18, < 0.21"
+nix = ">= 0.18, < 0.22"
 rand = "0.8.3"
 
 [dev-dependencies]