Changed fail macro to panic (compatibility with rust-lang/rust#17894)

src/cube.rs

@@ -42,7 +42,7 @@ impl Cube {
             3 => Quad::new(0b011, 0b111, 0b110, 0b010),
             4 => Quad::new(0b000, 0b010, 0b110, 0b100),
             5 => Quad::new(0b101, 0b111, 0b011, 0b001),
-            idx => fail!("{} face is higher then 6", idx)
+            idx => panic!("{} face is higher then 6", idx)
         }
     }