Auto merge of #50 - rphmeier:bump-heapsize, r=jdm

bump heapsize to 0.4.0

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Author: bors-servo

Cargo.toml

@@ -17,4 +17,4 @@ name = "smallvec"
 path = "lib.rs"
 
 [dependencies]
-heapsize = { version = "0.3", optional = true }
+heapsize = { version = "0.4", optional = true }