🛠 ignore bunyan updates for now (TryGhost#7637)

no issue
- background: we would like to pin 1.8.1 bunyan, because they introduced a behaviour change in the newer versions, see TryGhost/Ignition#16
- because we would like to use Ghost-Ignition in Ghost soon, we can ignore bunyan updates in Ghost for now
- Ignition will take care about be able to update bunyan soon

package.json

@@ -130,6 +130,7 @@
   },
   "greenkeeper": {
     "ignore": [
+      "bunyan",
       "glob",
       "mysql",
       "nodemailer",