Add support for CircleCI config version 2.1

[danielcompton]

CircleCI is creating version 2.1 for their config. https://github.com/CircleCI-Public/config-preview-sdk/blob/master/README.md. This isn't released yet, but it is in preview so people will start using it. I've opened a PR for this to add the 2.1 version in the existing config, although maybe it would be better for it to be a new schema entirely? Not quite sure.

[HafizAhmedMoon]

Any update on this issue?

I am getting an error on executor, which also accepts value map

[jth0024]

I'm also seeing this issue when using a when clause inside of workflows. See docs here