Bump half version

From half version 1.5.0 to 2.2.1

Author: BA8F0D39

Cargo.toml

@@ -52,11 +52,11 @@ afserde = ["serde"]
 libc = "0.2"
 num  = "0.4.0"
 lazy_static = "1.0"
-half = "1.5.0"
+half = { version = "2.2.1" , features = ["num-traits"] }
 serde = { version = "1.0", features = ["derive"], optional = true }
 
 [dev-dependencies]
-half = "1.5.0"
+half = { version = "2.2.1" , features = ["num-traits"] }
 serde_json = "1.0"
 bincode = "1.3"
 mnist = {version="0.5.0", features = ["download"]}