Bump version to 1.6.0
Bump version to 1.6.0

Summary: Updates the version to 1.6.0 ahead of a new release.

Reviewed By: lambdapioneer

Differential Revision: D5745178

fbshipit-source-id: ccbf477d861d0de1e9f6a14f385b48d1ff8d73bf

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