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.
Merge pull request #1 from jonathanpatt/wp-3.5-compatibility

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