Fixed crash when setting ORStackView.bottomMarginHeight without any child views on iOS8.

Adding a constraint with a `nil` view doesn't work anymore on iOS8. You get:

```
2014-06-06 18:13:13.759 ORStackViewExample[83043:1114513] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Unexpected use of internal layout attribute'
*** First throw call stack:
(
  0   CoreFoundation                      0x019a0916 __exceptionPreprocess + 182
  1   libobjc.A.dylib                     0x0163e8d9 objc_exception_throw + 44
  2   CoreFoundation                      0x019a07aa +[NSException raise:format:arguments:] + 138
  3   Foundation                          0x012bf67c -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 118
  4   UIKit                               0x0080628e -[UIView(AdditionalLayoutSupport) nsli_lowerAttribute:intoExpression:withCoefficient:forConstraint:] + 2452
  5   Foundation                          0x01234952 lower_1_attribute + 194
  6   Foundation                          0x012343cd -[NSLayoutConstraint _lowerIntoExpression:reportingConstantIsRounded:] + 99
  7   Foundation                          0x0122852f -[NSLayoutConstraint _addToEngine:integralizationAdjustment:mutuallyExclusiveConstraints:] + 97
  8   UIKit                               0x007f7ba3 -[UIView(UIConstraintBasedLayout) _layoutEngine_didAddLayoutConstraint:roundingAdjustment:mutuallyExclusiveConstraints:] + 508
  9   UIKit                               0x007f7ffb -[UIView(UIConstraintBasedLayout) _tryToAddConstraintWithoutUpdatingConstraintsArray:roundingAdjustment:mutuallyExclusiveConstraints:] + 61
  10  UIKit                               0x007f8148 -[UIView(UIConstraintBasedLayout) _tryToAddConstraint:roundingAdjustment:mutuallyExclusiveConstraints:] + 273
  11  UIKit                               0x007f8310 -[UIView(UIConstraintBasedLayout) _addConstraint:] + 234
  12  UIKit                               0x007f8221 -[UIView(UIConstraintBasedLayout) addConstraint:] + 40
  13  ORStackViewExampleTests             0x0c22eee4 -[UIView(FLKAutoLayoutPredicate) applyPredicate:toView:fromAttribute:toAttribute:] + 404
  14  ORStackViewExampleTests             0x0c22cf3f __78+[UIView(FLKAutoLayout) alignAttribute:ofViews:toAttribute:ofViews:predicate:]_block_invoke + 239
  15  ORStackViewExampleTests             0x0c229b1c -[FLKAutoLayoutPredicateList iteratePredicatesUsingBlock:] + 508
  16  ORStackViewExampleTests             0x0c22ccdc +[UIView(FLKAutoLayout) alignAttribute:ofViews:toAttribute:ofViews:predicate:] + 764
  17  ORStackViewExampleTests             0x0c22c95e +[UIView(FLKAutoLayout) alignAttribute:ofViews:toViews:predicate:] + 206
  18  ORStackViewExampleTests             0x0c22a6d5 -[UIView(FLKAutoLayout) alignAttribute:toView:predicate:] + 373
  19  ORStackViewExampleTests             0x0c22bd5c -[UIView(FLKAutoLayout) alignBottomEdgeWithView:predicate:] + 140
  20  ORStackViewExampleTests             0x0c23aa5b -[ORStackView updateConstraints] + 3083
  21  UIKit                               0x0080411c -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 272
  22  UIKit                               0x0080430f -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 127
  23  UIKit                               0x0080428a __UIViewRecursionHelper + 41
  24  CoreFoundation                      0x01892f69 CFArrayApplyFunction + 57
  25  UIKit                               0x008040ac -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 160
  26  UIKit                               0x0080430f -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 127
  27  UIKit                               0x0080428a __UIViewRecursionHelper + 41
  28  CoreFoundation                      0x01892f69 CFArrayApplyFunction + 57
  29  UIKit                               0x008040ac -[UIView(AdditionalLayoutSupport) _internalUpdateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 160
  30  UIKit                               0x0080430f -[UIView(AdditionalLayoutSupport) _updateConstraintsIfNeededAccumulatingViewsNeedingSecondPassAndViewsNeedingBaselineUpdate:] + 127
  31  UIKit                               0x008048a6 __60-[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded]_block_invoke + 105
  32  Foundation                          0x01238830 -[NSISEngine withBehaviors:performModifications:] + 143
  33  Foundation                          0x0123bda8 -[NSISEngine withAutomaticOptimizationDisabled:] + 48
  34  UIKit                               0x00803ecb -[UIView(AdditionalLayoutSupport) _withAutomaticEngineOptimizationDisabledIfEngineExists:] + 64
  35  UIKit                               0x008044fc -[UIView(AdditionalLayoutSupport) updateConstraintsIfNeeded] + 248
  36  UIKit                               0x007f710d -[UIWindow(UIConstraintBasedLayout) updateConstraintsIfNeeded] + 87
  37  UIKit                               0x007f705f -[UIWindow(UIConstraintBasedLayout) layoutSublayersOfLayer:] + 91
  38  libobjc.A.dylib                     0x01651763 -[NSObject performSelector:withObject:] + 70
  39  QuartzCore                          0x032aeb87 -[CALayer layoutSublayers] + 152
```