Fix the failing tests

Author: Stjepan Glavina

src/libcore/cell.rs

@@ -262,6 +262,8 @@ impl<T:Copy> Cell<T> {
     /// # Examples
     ///
     /// ```
+    /// #![feature(cell_update)]
+    ///
     /// use std::cell::Cell;
     ///
     /// let c = Cell::new(5);

src/libcore/tests/lib.rs

@@ -12,6 +12,7 @@
 
 #![feature(ascii_ctype)]
 #![feature(box_syntax)]
+#![feature(cell_update)]
 #![feature(core_float)]
 #![feature(core_private_bignum)]
 #![feature(core_private_diy_float)]