Update heapsize dependency.

Author: jdm

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "euclid"
-version = "0.12.0"
+version = "0.13.0"
 authors = ["The Servo Project Developers"]
 description = "Geometry primitives"
 documentation = "https://docs.rs/euclid/"
@@ -13,7 +13,7 @@ license = "MIT / Apache-2.0"
 unstable = []
 
 [dependencies]
-heapsize = "0.3"
+heapsize = "0.4"
 rustc-serialize = "0.3.2"
 num-traits = {version = "0.1.32", default-features = false}
 log = "0.3.1"