iOS自动布局解决警告Automatic Preferred Max Layout Width is not available on iOS versions prior to 8.0

部署环境为iOS7.0可能会遇到

设置Label的 Preferred Width(自动换行最大宽度)  explicit  (明确的)    即可

http://stackoverflow.com/questions/25398312/automatic-preferred-max-layout-width-is-not-available-on-ios-versions-prior-to-8