Fix compatibility issue with PHP 5.3

app/Core/Cache/Base.php

@@ -10,26 +10,6 @@ namespace Kanboard\Core\Cache;
  */
 abstract class Base
 {
-    /**
-     * Fetch value from cache
-     *
-     * @abstract
-     * @access public
-     * @param  string  $key
-     * @return mixed            Null when not found, cached value otherwise
-     */
-    abstract public function get($key);
-
-    /**
-     * Save a new value in the cache
-     *
-     * @abstract
-     * @access public
-     * @param  string  $key
-     * @param  mixed   $value
-     */
-    abstract public function set($key, $value);
-
     /**
      * Proxy cache
      *