Fix RETableViewBoolCell constraints on iOS 8

On iOS 8 UIKit adds own constraints to contentView, so removing all
contentView’s constraints causes problems when the cell
is displayed for the second time on the screen.
Fix RETableViewSegmentedCell constraints on iOS 8

On iOS 8 UIKit adds own constraints to contentView, so removing all
contentView’s constraints causes problems when the cell
is displayed for the second time on the screen.
Merge pull request #178 from fastred/master

Fix RETableViewBoolCell's and RETableViewSegmentedCell's layout on iOS 8