Fix for create_function deprecated in PHP 7.2

A couple of lines in ultimate-posts-widget.php need to be changed to use anonymous functions.

So line 154 should go from this:
$new_excerpt_more = create_function('$more', 'return "...";');
to this:

$new_excerpt_more = function($more) {
     return '...';
};

And line 158 should go from this:
$new_excerpt_length = create_function('$length', "return " . $excerpt_length . ";");
to this:

$new_excerpt_length = function($length) {
    return ' . $excerpt_length . ';
};