Use serde_json for target spec json
diff --git a/Cargo.lock b/Cargo.lock
index 7ed327e..aefaaad 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3774,6 +3774,7 @@
  "rustc_span",
  "rustc_target",
  "rustc_typeck",
+ "serde_json",
  "tracing",
  "winapi",
 ]
@@ -4445,6 +4446,7 @@
  "rustc_macros",
  "rustc_serialize",
  "rustc_span",
+ "serde_json",
  "tracing",
 ]