Replace Cisaurus slug copy with v3 Releases API

This replaces the use of Cisaurus in `heroku fork`
with the v3 slug copy. This should make fork faster
and remove the dependency on Cisaurus, which is being
deprecated.
Remove extraneous debug line with assoicated test fix
Merge pull request #1122 from heroku/fork-v3-api

Replace Cisaurus slug copy with v3 Releases API