[Issue #221] Fix a bug that can't get triggers from Zabbix 2.2.0 - 2.2.1

Hatohol can't get triggers from Zabbix 2.2.0 and 2.2.1 because these versions have
a bug that the name of a property of trigger object is wrong.

True:  value_flags
False: value_flag

In addition "value_flags" is a deprecated property on Zabbix 2.2 series.
So this fix ignore the property.