Update CF fetcher to follow Heroku updates.

Heroku change introduced in https://github.com/heroku/heroku-buildpack-ruby/commit/496f799e1465f7ed050015e4518507a4e47edc38#diff-d41d8cd98f00b204e9800998ecf8427e added a second parameter to fetch_untar. Our fetch_untar was incompatible with the change.

[#77843004]