Changed order of Ghosthawk turret classes

With my own mod that used all 3 turret positions in Ghosthawk Copilot
and Left Gun were inversed because of the order their classes were put
down in config.cpp. This fixes it and adds compatibility with my mod and
hopefully others getting the same incompatibility problem.
Merge pull request #440 from jonpas/master

Changed order of Ghosthawk turret classes