Avoid new validation in proc-macro2

Evidently, we run afoul of this.

Author: tamird

Cargo.toml

@@ -53,7 +53,9 @@ peeking_take_while = "0.1.2"
 quote = { version = "0.5", default-features = false }
 regex = "0.2"
 which = "1.0.2"
-proc-macro2 = { version = "0.3.2", default-features = false }
+# New validation in 0.3.6 breaks bindgen-integration:
+# https://github.com/alexcrichton/proc-macro2/commit/489c642.
+proc-macro2 = { version = "0.3.2, < 0.3.6", default-features = false }
 
 [dependencies.env_logger]
 optional = true