[README] Fix notation of Zabbix.

The official notation is "Zabbix" not "ZABBIX".
[README] Update supported monitoring software.

Zabbix 2.4 is already supported. Please see: #197
Nagios4 is supported with NDOUtils version 2.0.
[README] Synchronize supported monitoring software in README at server
directory.
[README] Add supported incident tracking software.
[README] Fix version number of Zabbix 2.4.

I was wrong.
[README] Fix link of Installation and Upgrade documents.
Merge pull request #756 from project-hatohol/update-README-version

Update README