Summary:
adopt adjustedContentInset instead of contentInset on iOS 11
- [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/1020
Reviewed By: manicakes
Differential Revision: D6513703
Pulled By: rnystrom
fbshipit-source-id: 5d718f2cb30361959ca5ba8238746427223cb5fd