Update cocoa version to 0.18.4

Author: pcwalton

cocoa/Cargo.toml

@@ -4,7 +4,7 @@ name = "cocoa"
 description = "Bindings to Cocoa for OS X"
 homepage = "https://github.com/servo/core-foundation-rs"
 repository = "https://github.com/servo/core-foundation-rs"
-version = "0.18.3"
+version = "0.18.4"
 authors = ["The Servo Project Developers"]
 license = "MIT / Apache-2.0"