[ArmZabbixAPI] Don't read removed properties on Zabbix 2.4

* trigger.value_falgs
* item.filter
[JsonParseAgent] Check return value of json_node_get_string() to avoid crash
[testArmZabbixAPI] Add tests for getting items & triggers of Zabbix 2.3

Zabbix 2.3 is a development version and the corresponding stable
version is 2.4, but the stable version isn't released yet. After
it's released we should recheck these implementations.
Merge pull request #198 from project-hatohol/zabbix-2.4

Ready for Zabbix 2.4