Limit stale checks to issues

[bkeepers]

@rafaelfranca, probot/stale#8 adds support for marking Pull Requests as stale in addition to Issues. The default will be to check both Issues and PRs, so I updated the config here to limit stale checking to issues.

Feel free to just close this PR If you want stale checks on PRs as well, but I will wait to hear from you either way before I deploy probot/stale#8.

[rails-bot]

Thanks for the pull request, and welcome! The Rails team is excited to review your changes, and you should hear from @kaspth (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

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[maclover7]

r? @rafaelfranca

[matthewd]

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