Changes in version number

• Changed the return type of Zabbix.api_version() from String to a more Julia centric VersionNumber.
  So now the code looks like,

zabbix = Zabbix.ZabbixAPI("http://SERVER_IP/zabbix/api_jsonrpc.php","username","password")

julia> Zabbix.make_request(zabbix, "apiinfo.version", Dict())
v"3.2.11"

julia> typeof(Zabbix.api_version(zabbix))
VersionNumber