Pin proc_macro2 dependency to v1.0.32

Author: chordtoll

Cargo.lock

@@ -21,6 +21,7 @@ log = "0.4"
 semver = "1.0" # Keep in sync with version pulled by Cargo
 serde = { version = "1.0.130", features = ["derive"] }
 serde_json = "1.0.72"
+proc-macro2 = "=1.0.32"
 
 [dev-dependencies]
 quickcheck = { version = "1.0", default-features = false }