Added proper cache::forever implementations to each driver.

Signed-off-by: Taylor Otwell <taylorotwell@gmail.com>

laravel/cache/drivers/memcached.php

@@ -71,6 +71,18 @@ class Memcached extends Driver {
 		$this->memcache->set($this->key.$key, $value, 0, $minutes * 60);
 	}
 
+	/**
+	 * Write an item to the cache that lasts forever.
+	 *
+	 * @param  string  $key
+	 * @param  mixed   $value
+	 * @return void
+	 */
+	public function forever($key, $value)
+	{
+		return $this->put($key, $value, 0);
+	}
+
 	/**
 	 * Delete an item from the cache.
 	 *