[misc] install bunyan as production dependency

```
Error: Cannot find module 'bunyan'
Require stack:
- .../node_modules/@google-cloud/logging-bunyan/build/src/middleware/express.js
- .../node_modules/@google-cloud/logging-bunyan/build/src/index.js
- .../node_modules/logger-sharelatex/logging-manager.js
- .../node_modules/logger-sharelatex/index.js
- .../app.js
```

services/spelling/package.json

@@ -28,6 +28,7 @@
     "@overleaf/o-error": "^3.0.0",
     "async": "^2.6.3",
     "body-parser": "^1.19.0",
+    "bunyan": "^1.8.15",
     "coffee-script": "^1.12.7",
     "express": "^4.17.1",
     "logger-sharelatex": "^2.2.0",
@@ -38,7 +39,6 @@
     "underscore": "1.13.1"
   },
   "devDependencies": {
-    "bunyan": "^1.8.12",
     "chai": "4.2.0",
     "eslint": "^6.8.0",
     "eslint-config-prettier": "^6.10.0",