Fix rb-sys version (#60)

Author: jarkonik

Cargo.toml

@@ -32,7 +32,7 @@ mlua = { version = "0.9.8", features = [
     "send",
 ], optional = true }
 magnus = { version = "0.7.1", optional = true }
-rb-sys = { version = "*", default-features = false, features = ["link-ruby", "ruby-static"], optional = true }
+rb-sys = { version = "0.9", default-features = false, features = ["link-ruby", "ruby-static"], optional = true }
 crossbeam-channel = "0.5.15"
 libc = "0.2.172"