Upgrade travis xcode image to 6.4
Upgrade travis xcode image to 6.4

cloudant/CDTDatastore,phatblat/CDTDatastore,phatblat/CDTDatastore,jhs/CDTDatastore,phatblat/CDTDatastore,cloudant/CDTDatastore,jhs/CDTDatastore,jhs/CDTDatastore,cloudant/CDTDatastore,cloudant/CDTDatastore