Temporarily disable probot

https://github.com/exercism/xjava/issues/555#issuecomment-304126241

.github/stale.yml

-# Issues or Pull Requests with these labels will never be considered stale
-exemptLabels:
-  - "first-timers only"
-  - "good first patch"
-  - "policy"
-
-# Label to use when marking as stale
-staleLabel: stale