Remove extern in src/constants.rs (#421)

We try to keep all externs to either bin/cernan.rs or src/lib.rs.
This commit removes an extern inside of the cernan library.

Signed-off-by: Brian L. Troutwine <brian@troutwine.us>

src/constants.rs

@@ -1,6 +1,5 @@
 //! Library level constants
-
-extern crate mio;
+use mio;
 
 /// MIO token used to distinguish system events
 /// from other event sources.