Test for heroku-postgresql addon before copying pg data

Test to see if heroku-postgresql addon is enabled on the source app before trying to copy pg data to the new forked app.
You are not asserting that it's not called here.

You should do, I think:

```ruby
Fourchette::Pgbackups.any_instance.should_not_receive(:copy)
```

Removing lines 137 to 140 included, plus 142.