fix build failure on OS X Yosemite
fix build failure on OS X Yosemite

yfinkelstein/node-zookeeper,mdlavin/node-zookeeper,mdlavin/node-zookeeper,mdlavin/node-zookeeper,mdlavin/node-zookeeper,dfsxdev/node-zookeeper,dfsxdev/node-zookeeper,dfsxdev/node-zookeeper,mdlavin/node-zookeeper,yfinkelstein/node-zookeeper,yfinkelstein/node-zookeeper,yfinkelstein/node-zookeeper,dfsxdev/node-zookeeper,dfsxdev/node-zookeeper