Remove now-stable alloc feature gate

Author: phil-opp

Changelog.md

@@ -1 +1,2 @@
 - Align up the Hole initialization address ([#18](https://github.com/phil-opp/linked-list-allocator/pull/18))
+- Remove `alloc` feature gate, which is now stable

src/lib.rs

@@ -1,5 +1,5 @@
 #![feature(const_fn)]
-#![feature(alloc, allocator_api)]
+#![feature(allocator_api)]
 #![no_std]
 
 #[cfg(test)]