Fixed autolayout issues

I changed line 79 from
```objc
CGFloat height = MAX(0, -y + maxY);
```
to
```objc
CGFloat height = MAX(1, -y + maxY);
```
to prevent an issue with autolayout:
```
2014-02-15 16:51:44.073 CSStickyHeaderFlowLayoutDemo[14879:70b] Unable to simultaneously satisfy constraints.
	Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x14a40e90 h=-&- v=-&- UIView:0x9ae7620.height == UICollectionViewCell:0x9a23ee0.height - 0.5>",
    "<NSLayoutConstraint:0x9d4aa10 'UIView-Encapsulated-Layout-Height' V:[UICollectionViewCell:0x9a23ee0(0)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x9d4aa10 'UIView-Encapsulated-Layout-Height' V:[UICollectionViewCell:0x9a23ee0(0)]>

Break on objc_exception_throw to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.
```