Fix app crashing when applying auto layout on tabbaritems.

some updates for supporting auto-layout animation

On iOS below 8 (7.1;7.0;Sim or Real device), app will crash and prompt to call [super layoutSubviews]
*** Assertion failure in -[RDVAppTabBar layoutSublayersOfLayer:], /SourceCache/UIKit_Sim/UIKit-2935.137/UIView.m:8794
*** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: 'Auto Layout still required after executing -layoutSubviews. RDVAppTabBar's implementation of -layoutSubviews needs to call super.'

p.s. This fix will not affect non-autolayout view's display.