Release 5.2.1

Merge pull request IBAnimatable#558 from IBAnimatable/release/5.1.0

Cut release 5.1.0 #no-public-changes

Update the `Key Features` in README.md

Get release 4.2.0 ready.

Merge pull request IBAnimatable#475 from IBAnimatable/feature/get-4-1…

…-ready

Get release 4.1.0 ready

Merge pull request IBAnimatable#454 from IBAnimatable/feature/release…

…_400

Bump 4.0.0

Add contributor to the CHANGELOG.md

Get release 3.1.2 ready.

Prepare for release 3.1.1