Summary:
Added `-didDeselectSectionController:withObject:` to `IGListSingleSectionControllerDelegate`.
Not sure if it makes sense to make it non-optional in 4.0.0 or not (#909) - I can add the [same note](0f04a07319/Source/IGListBindingSectionControllerSelectionDelegate.h (L40)) to the docs if it is
- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)
Closes https://github.com/Instagram/IGListKit/pull/954
Differential Revision: D6164711
Pulled By: rnystrom
fbshipit-source-id: aa8cfd2bb72a16cb525d875e2cad93888f13c641
