Updated powermock version to 1.6.6
Updated powermock version to 1.6.6

Summary: This fixes failing `AnimatedImageFactoryGifImplTest` on Windows.

Reviewed By: kirwan

Differential Revision: D4461894

fbshipit-source-id: e17410ab1a91958c99b23d0ea94f0911c99e6864

desmond1121/fresco,xjy2061/NovaImageLoader,desmond1121/fresco,MaTriXy/fresco,MaTriXy/fresco,facebook/fresco,MaTriXy/fresco,s1rius/fresco,s1rius/fresco,facebook/fresco,s1rius/fresco,xjy2061/NovaImageLoader,desmond1121/fresco,facebook/fresco,MaTriXy/fresco,facebook/fresco,desmond1121/fresco,xjy2061/NovaImageLoader,xjy2061/NovaImageLoader,xjy2061/NovaImageLoader,facebook/fresco,facebook/fresco,MaTriXy/fresco,s1rius/fresco,desmond1121/fresco