Pin proc-macro-hack to identical version of impl crate

Author: dtolnay

Cargo.toml

@@ -7,7 +7,7 @@ description = "Procedural functionlike!() macros using only Macros 1.1"
 repository = "https://github.com/dtolnay/proc-macro-hack"
 
 [dependencies]
-proc-macro-hack-impl = { version = "0.4", path = "impl" }
+proc-macro-hack-impl = { version = "=0.4.2", path = "impl" }
 
 [dev-dependencies]
 demo-hack = { version = "0.0.4", path = "demo-hack" }