Known civ bonus based on most advanced player era

Known civ bonus is changed so that it is based on the most advanced player era with following equation:
 HighestPlayerEra*TECH_COST_TOTAL_KNOWN_TEAM_MODIFIER
 0 = ERA_ANCIENT
 1 = ERA_CLASSICAL
 2 = ERA_MEDIEVAL
 3 = ERA_RENAISSANCE
 4 = ERA_INDUSTRIAL
 5 = ERA_MODERN
 6 = ERA_FUTURE