Bind value in wpdb::prepare() for WP 3.5

As of WordPress 3.5, wbdb::prepare() enforces a minimum of 2 arguments,
presumably in an attempt to ensure its proper use by binding values to
the SQL statement. This commit binds the $current_date variable.

See the *Protect Queries Against SQL Injection Attacks* section of
[Class Reference/wpdb](https://codex.wordpress.org/Class_Reference/wpdb).