Import std::num::Num explicitly.

It was removed from the prelude as part of <rust-lang/rust#18827>.

Author: Ms2ger

src/side_offsets.rs

@@ -10,7 +10,7 @@
 //! A group of side offsets, which correspond to top/left/bottom/right for borders, padding,
 //! and margins in CSS.
 
-use std::num::{NumCast, Zero};
+use std::num::{Num, NumCast, Zero};
 
 /// A group of side offsets, which correspond to top/left/bottom/right for borders, padding,
 /// and margins in CSS.