Update ServerState.java

[INFO] Compilation failure
/home/ec2-user/highcharts.com/exporting-server/java/highcharts-export/src/main/java/com/highcharts/export/pool/ServerObjectFactory.java:[68,29] 找不到符号
符号： 变量 ACTIVE
位置： 类 com.highcharts.export.server.ServerState