Bug: AI Cities produces the only Caravans after Settlers

[iegik]

https://github.com/fire-eggs/CivOne/blob/19eef7d8e9250afcb46fc0be629fcf2dd107c51f/src/AI.cs#L266-L288

Need to determinate when and how match Caravans to build.

I think, that Cities need 3 trade routes for each city to support economy. This may vary against of difficulty level. Correct me if I'm wrong.

Related with https://github.com/fire-eggs/CivOne/issues/169 https://github.com/fire-eggs/CivOne/issues/165