[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/notifications/package-lock.json

@@ -1645,13 +1645,12 @@
       "integrity": "sha512-lGzLKcioL90C7wMczpkY0n/oART3MbBa8R9OFGE1rJxoVI86u4WAGfEk8Wjv10eKSyTHVGkSo3bvBylCEtk7LA=="
     },
     "bunyan": {
-      "version": "1.8.12",
+      "version": "1.8.15",
-      "resolved": "https://registry.npmjs.org/bunyan/-/bunyan-1.8.12.tgz",
+      "resolved": "https://registry.npmjs.org/bunyan/-/bunyan-1.8.15.tgz",
-      "integrity": "sha1-8VDw9nSKvdcq6uhPBEA74u8RN5c=",
+      "integrity": "sha512-0tECWShh6wUysgucJcBAoYegf3JJoZWibxdqhTm7OHPeT42qdjkZ29QCMcKwbgU1kiH+auSIasNRXMLWXafXig==",
-      "dev": true,
       "requires": {
         "dtrace-provider": "~0.8",
-        "moment": "^2.10.6",
+        "moment": "^2.19.3",
         "mv": "~2",
         "safe-json-stringify": "~1"
       }

services/notifications/package.json

@@ -20,6 +20,7 @@
     "@overleaf/metrics": "^3.5.1",
     "async": "^2.6.3",
     "body-parser": "^1.19.0",
+    "bunyan": "^1.8.15",
     "coffee-script": "^1.12.7",
     "errorhandler": "^1.5.1",
     "express": "4.17.1",
@@ -32,7 +33,6 @@
   },
   "devDependencies": {
     "babel-eslint": "^10.1.0",
-    "bunyan": "^1.8.12",
     "chai": "^4.2.0",
     "eslint": "^6.8.0",
     "eslint-config-prettier": "^6.10.0",