add bug label to stale.yml

Author: reconbot

.github/stale.yml

@@ -7,6 +7,7 @@ exemptLabels:
   - "Up for consideration"
   - greenkeeper
   - neverstale
+  - bug
 # Label to use when marking an issue as stale
 staleLabel: stale
 # Comment to post when marking an issue as stale. Set to `false` to disable