[server][ItemData] Overload a cast operator: ItemInt::operator const bool &().

This is a trial to fix a failure on TravisCI:
https://travis-ci.org/project-hatohol/hatohol/jobs/17978470

The following is a message when the problem happened:
  testArmZabbixAPI::test_makeItemVecotr:		terminate called after throwing an instance of 'ItemDataException'
  what():  <../src/ItemData.h:217> Undefined operation: 'cast to bool' (Integer) (ItemID: 1)