Bump cargo_metadata to 0.8

Author: tesuji

Cargo.toml

@@ -34,7 +34,7 @@ required-features = ["rustc_tests"]
 
 [dependencies]
 byteorder = { version = "1.1", features = ["i128"]}
-cargo_metadata = { version = "0.7", optional = true }
+cargo_metadata = { version = "0.8", optional = true }
 directories = { version = "1.0", optional = true }
 rustc_version = { version = "0.2.3", optional = true }
 env_logger = "0.6"