iOS8ではconstraintにuser define runtime attributeに値を設定しても落ちないが、iOS7では落ちる


間違えてstroyboardのuser define runtime attributeに値を追加してしまったときに、気づきにくいバグになってしまう。
エラーのメモがてら記録しておきます。

```
2014-10-20 18:37:10.180 app-qa[44937:70b] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<NSLayoutConstraint 0x7ff527238d90> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key keyPath.'
*** First throw call stack:
(
	0   CoreFoundation                      0x000000010c1e1795 __exceptionPreprocess + 165
	1   libobjc.A.dylib                     0x000000010bf3d991 objc_exception_throw + 43
	2   CoreFoundation                      0x000000010c265e69 -[NSException raise] + 9
	3   Foundation                          0x0000000109fb503a -[NSObject(NSKeyValueCoding) setValue:forKey:] + 259
	4   CoreFoundation                      0x000000010c1dd700 -[NSArray makeObjectsPerformSelector:] + 224
	5   UIKit                               0x000000010b0ea1a5 -[UINib instantiateWithOwner:options:] + 1112
	6   UIKit                               0x000000010af8c4a1 -[UIViewController _loadViewFromNibNamed:bundle:] + 245
	7   UIKit                               0x000000010af8cade -[UIViewController loadView] + 112
	8   UIKit                               0x000000010af8cdfd -[UIViewController loadViewIfRequired] + 75
	9   UIKit                               0x000000010af8d1bd -[UIViewController view] + 29
	10  UIKit                               0x000000010afa37a5 -[UINavigationController _startCustomTransition:] + 628
	11  UIKit                               0x000000010afae9b5 -[UINavigationController _startDeferredTransitionIfNeeded:] + 401
	12  UIKit                               0x000000010afaf4f8 -[UINavigationController __viewWillLayoutSubviews] + 43
	13  UIKit                               0x000000010b0c37e1 -[UILayoutContainerView layoutSubviews] + 202
	14  UIKit                               0x000000010aefcb27 -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 354
	15  QuartzCore                          0x000000010abcfa22 -[CALayer layoutSublayers] + 151
	16  QuartzCore                          0x000000010abc4589 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 363
	17  QuartzCore                          0x000000010abc440a _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 24
	18  QuartzCore                          0x000000010ab39694 _ZN2CA7Context18commit_transactionEPNS_11TransactionE + 252
	19  QuartzCore                          0x000000010ab3a70c _ZN2CA11Transaction6commitEv + 394
	20  QuartzCore                          0x000000010ab3ad79 _ZN2CA11Transaction17observer_callbackEP19__CFRunLoopObservermPv + 89
	21  CoreFoundation                      0x000000010c1acff7 __CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
	22  CoreFoundation                      0x000000010c1acf67 __CFRunLoopDoObservers + 391
	23  CoreFoundation                      0x000000010c18c6d2 __CFRunLoopRun + 946
	24  CoreFoundation                      0x000000010c18bf33 CFRunLoopRunSpecific + 467
	25  GraphicsServices                    0x000000010e6e23a0 GSEventRunModal + 161
	26  UIKit                               0x000000010aea1043 UIApplicationMain + 1010
	27  app-qa                              0x000000010807413b main + 155
	28  libdyld.dylib                       0x000000010c7a05fd start + 1
	29  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException
```