Summary:
## Changes in this pull request
This updates the Podfile to use the specs CDN instead of the git checkout, which is more performant (and the default with CocoaPods 1.8 and higher).
### Checklist
- [x] All tests pass. Demo project builds and runs.
- [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)
Pull Request resolved: https://github.com/Instagram/IGListKit/pull/1386
Reviewed By: lorixx
Differential Revision: D18572204
Pulled By: iperry90
fbshipit-source-id: d3b721aaff92b18efeef31282cb9204055baec21
