GlobalTransport: Truncate order_id

From their docs:

http://cl.ly/image/2H0h1X2o2N3N/content.png

They allow 25 characters for InvNum for all cards except Amex for which
they allow only 16 characters.  I'm not sure why they'd distinguish
based on card brand.

We'll truncate to 16 characters for this gateway to be consistent across
the board.  Hopefully this will help customers find out sooner rather
than later that the max is 16 for some card types.