Fix air transports refusing to unload Skuttle and Tick

Air transports (both Valkyrie and Vindicator) were unable to unload Skuttle or Tick, when playing in Spring 91.0 (but not in latest 97.0+). They just froze in place right when they tried to place the unit on ground.

I investigated the issue and it proved to be caused be too low model height: 4 for Skuttle and 4.76 for Tick. 
Testing showed that minimal model height must be about 6.0 for unloading to work at all and about 6.5 to work reliably (unload immediately and not after a few seconds of trying). Changing the height of already created unit using Spring.SetUnitRadiusAndHeight() didn't help, only changing the height in the model file did.

No other loadable unit except these two had model height low enough to cause problems.