Fix unused extern crate error

Author: belltoy

Cargo.toml

@@ -21,9 +21,11 @@ regex = { version = "0.2", optional = true }
 slog-term = "2"
 slog-stdlog = "2"
 slog-scope = "3"
-slog-async = "2"
 log = "0.3.6"
 
+[dev-dependencies]
+slog-async = "2"
+
 [[test]]
 name = "regexp_filter"
 harness = false

src/lib.rs

@@ -76,7 +76,6 @@ extern crate slog;
 extern crate slog_term;
 extern crate slog_stdlog;
 extern crate slog_scope;
-extern crate slog_async;
 extern crate log;
 
 use std::{env, result, sync};