AI shouldn't be building navies in undefended cities, especially with barbarians nearby

[QuintillusCFC]

I hope Larsa's governor struck a deal with these barbarians, because it doesn't look good:

This probably requires a tweak to weighting in CityProductionAI (GetNextItemToBeProduced and methods it calls). I don't think naval units are currently getting nerfed in this situation, but they should be.