Don't add staging/production remotes w/o access

If user doesn't have access to staging or production Heroku applications, they
do not need/want to have the Git remotes created.

Real-world example:

https://github.com/codecation/trailmix/pull/123
It hasn't been moved to a template file due to the Heroku bits being controlled by a command line flag and the content needing to be interpolated as a template through ERb, then appended to the existing `bin/setup` file. There's enough moving components in a pattern we haven't used yet that I haven't had a chance to really sit down and figure it all out. I'd like to merge this as-is and leave that exercise as a refactoring for myself or someone else rather than letting this PR continue to get stale.