Prepare 4.0.1 Release (AFNetworking#4555)

* Version to 4.0.1.

* Update Gems.

Prepare 4.0.0. (AFNetworking#4527)

Update CHANGELOG.md

3.2.1 (fastlane)

Merge pull request AFNetworking#4138 from AFNetworking/unguarded-avai…

…lability

Use @available if available to silence a warning.

3.1.0 (fastlane)

3.0.4 (fastlane)

3.0.3 (fastlane)

3.0.2 (fastlane)

3.0.1 (fastlane)