Update nix from 0.30.1 to 0.31.3

Author: thehouseisonfire

Cargo.toml

@@ -16,7 +16,7 @@ categories = ["filesystem"]
 rand = { version = "0.10.2" }
 
 [target.'cfg(unix)'.dependencies]
-nix = { version = "0.30.1", features = ["fs", "user"] }
+nix = { version = "0.31.3", features = ["fs", "user"] }
 
 [features]
 default = []