Fix failing build

This is a bug in... Something. Clippy folks are looking into it.

Author: sgrif

diesel_cli/src/config.rs

@@ -1,3 +1,6 @@
+// FIXME: https://github.com/rust-lang-nursery/rust-clippy/issues/2910
+#![cfg_attr(feature = "clippy", allow(useless_attribute))]
+
 use clap::ArgMatches;
 use std::env;
 use std::error::Error;