Application will crash if one of the content view constraint is a NSAutoresizingMaskLayoutConstraint

If one of the constraint is not a vanilla `NSLayoutConstraint` but a `NSAutoresizingMaskLayoutConstraint` instance, the application will crash in the `-[BMXSwipable bmx_moveContentViewSubViews]` method when creating the new contraint.

This is occurs with iOS 8, even if the auto layout is used and no translation for autoresizing mask to contraints is set.

The present fix will avoid such crash.