Specify required version of cargo_metadata

Author: Rob Tsuk

Cargo.toml

@@ -47,7 +47,7 @@ log = "0.4"
 env_logger = "0.5"
 getopts = "0.2"
 derive-new = "0.5"
-cargo_metadata = "0.5"
+cargo_metadata = "0.5.1"
 rustc-ap-syntax = "60.0.0"
 rustc-ap-rustc_errors = "60.0.0"