fix build failure on OS X Yosemite

Fixed #99 

There is a function name conflict between Zookeeper and Yosemite, and there is a patch to Zookeeper. So I modified the BASH file to download that patch first and then apply it if workstation is Yosemite.

See https://issues.apache.org/jira/browse/ZOOKEEPER-2049 for more details.