Update json crate to 0.12

Author: dtolnay

Cargo.toml

@@ -8,7 +8,7 @@ publish = false
 [dependencies]
 getopts = "0.2"
 jemallocator = "0.3"
-json = { version = "0.11", optional = true }
+json = { version = "0.12", optional = true }
 rustc-serialize = { version = "0.3", optional = true }
 serde = { version = "1.0", features = ["derive"], optional = true }
 serde-bench = { version = "0.0.7", optional = true }