Extend serde_json version range

Author: sfackler

Cargo.toml

@@ -30,7 +30,7 @@ net2 = "0.2.16"
 rustc-serialize = { version = "0.3", optional = true }
 chrono = { version = "0.2.14", optional = true }
 openssl = { version = ">= 0.6.4, < 0.8", optional = true }
-serde_json = { version = "0.6", optional = true }
+serde_json = { version = ">= 0.6, < 0.8", optional = true }
 time = { version = "0.1.14", optional = true }
 unix_socket = { version = "0.5", optional = true }
 uuid = { version = "0.1", optional = true }