@orta => Fix snapshot tests

I forked an [existing fork](https://github.com/facebook/ios-snapshot-test-case/pull/46) of the snapshot library and am using  [branch of Nimble-Snapshots](https://github.com/AshFurrow/Nimble-Snapshots/compare/snapshot-fix?expand=1) – pretty simple changes. 

This PR just updates the submodule and pod locations to point to my fork/branch and updates the reference images (because there are sub-pixel level differences between the two rendering methods).