Fix compilation errors when using XCode 8.1
Fix compilation errors when using XCode 8.1

Summary:
Fix for https://github.com/facebook/WebDriverAgent/issues/374
Closes https://github.com/facebook/WebDriverAgent/pull/383

Differential Revision: D4234092

Pulled By: marekcirkos

fbshipit-source-id: 1237da33d675f1de093c704386b4cf1894aca7dc

ymin/WebDriverAgent,Gogle/WebDriverAgent,b1ueshad0w/WebDriverAgent,riccoyu/WebDriverAgent,riccoyu/WebDriverAgent,riccoyu/WebDriverAgent,zenzhu/WebDriverAgent,drptbl/WebDriverAgent,b1ueshad0w/WebDriverAgent,zenzhu/WebDriverAgent,qa-dev/WebDriverAgent,PiranhaTeam/WebDriverAgent,riccoyu/WebDriverAgent,qa-dev/WebDriverAgent,ymin/WebDriverAgent,Gogle/WebDriverAgent,drptbl/WebDriverAgent,PiranhaTeam/WebDriverAgent,b1ueshad0w/WebDriverAgent,qa-dev/WebDriverAgent,b1ueshad0w/WebDriverAgent,drptbl/WebDriverAgent,ymin/WebDriverAgent,ymin/WebDriverAgent,calabash/WebDriverAgent,PiranhaTeam/WebDriverAgent,calabash/WebDriverAgent,calabash/WebDriverAgent,zenzhu/WebDriverAgent,PiranhaTeam/WebDriverAgent,PiranhaTeam/WebDriverAgent,Gogle/WebDriverAgent,Gogle/WebDriverAgent,qa-dev/WebDriverAgent