Teach CLI to figure out a default Heroku Postgres plan for fork/follow/rollback

Default to primary database plan if no plan specified

Good to default to what you already have if you don't specify a :plan
Collapse resize+scale into a single API call with V3.

The current heroku-api gem only supports V2, so we make a raw HTTP
request with Excon here, but it's not bad.
Collapse do_resize back into the resize method.

It's no longer called from ps:scale, so it can go back to being inline.
Don't turn quantity into an integer; it can contain relative values.
Attempt to stub out V3 formation API.

This seems to have no effect.
Use Excon.stub instead of Webmock.

Couldn't figure out why Webmock refused to apply to the V3 API calls
but works fine for the V2 ones, but Excon.stub works.
Use bulk scale API to make changes to all dyno types simultaneously.

The response now includes size/quantity for all known process types
rather than just the changed one, so we currently output all of them.
Merge pull request #954 from catsby/default_rollback

Teach CLI to figure out a default Heroku Postgres plan for fork/follow/rollback
Go back to only emitting the names of the processes that were actually scaled.
Merge branch 'resize-in-scale'

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v3.1.1