Allow serde 0.8

Cargo.toml

@@ -36,4 +36,4 @@ heapsize_plugin = {version = "0.1.0", optional = true}
 idna = { version = "0.1.0", path = "./idna" }
 matches = "0.1"
 rustc-serialize = {version = "0.3", optional = true}
-serde = {version = ">=0.6.1, <0.8", optional = true}
+serde = {version = ">=0.6.1, <0.9", optional = true}