Disable length_delimited deprecation warning. (#321)

Until tokio-rs/tokio#680 is resolved, we should allow using deprecated
APIs in the codec module.

Author: carllerche

src/codec/mod.rs

@@ -1,3 +1,6 @@
+// Until tokio-rs/tokio#680 is fixed
+#![allow(deprecated)]
+
 mod error;
 mod framed_read;
 mod framed_write;