add --active-only option to list command

[kellyselden]

We need a way to get the active revision, no matter how old it is. I don't think this is possible now, because you would need to know the amount beforehand. Right now we are working around the issue with:

    'display-revisions': {
      revisions(context) {
        if (process.env.ACTIVE_ONLY) {
          context.revisions = context.revisions.filter(revision => revision.active);
        }

        return context.revisions;
      },
    },

but we would like to upstream this behaviour.

Implementation PR ember-cli-deploy/ember-cli-deploy-display-revisions#23.

[lukemelia]

@kellyselden I understand the need and this looks fine to me. In order to be merged, this PR should include:

• Documentation update (at least here: https://github.com/ember-cli-deploy/ember-cli-deploy/blob/4866e3aeaaf51c7623acf3c0e1b31310d6d9b4e5/docs/docs/v1.0.x/deploying-your-app.md#listing)

We'll also need to:

• Resolve unrelated travis build issues