Update CHANGELOG.md
Update CHANGELOG.md

Summary:
Per our decision to drop the 2.2 release because of our internal syncing system issues.

Updates changelog to remove `2.2` and move entries to `3.0`
Closes https://github.com/Instagram/IGListKit/pull/512

Differential Revision: D4609080

Pulled By: jessesquires

fbshipit-source-id: e60a2d0534989d8ae1a65b493b14898beee2be6a

Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,Instagram/IGListKit,Instagram/IGListKit,Instagram/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit,zhubofei/IGListKit