Add clippy::mod_module_files lint (#136)

Lints against using the 2015 module style.

Fixes one lingering instance of this style.

Author: tarcieri

src/lib.rs

@@ -7,6 +7,7 @@
 )]
 #![deny(unsafe_code)]
 #![warn(
+    clippy::mod_module_files,
     clippy::unwrap_used,
     missing_docs,
     missing_debug_implementations,