fix: move jsonwebtoken to dev dependencies (#677)

jsonwebtoken was only used in tests and not present in the release output.

```
[0] firebase-functions$ ag jsonwebtoken
spec/fixtures/mockrequest.ts
1:import * as jwt from 'jsonwebtoken';

package.json
50:    "@types/jsonwebtoken": "^8.3.2",
64:    "jsonwebtoken": "^8.5.1",
[0] firebase-functions$ grep jsonwebtoken -R lib
[1] firebase-functions$
```

Co-authored-by: Lauren Long <laurenzlong@users.noreply.github.com>

Author: sk-

package.json

@@ -41,7 +41,6 @@
     "@types/express": "4.17.3",
     "cors": "^2.8.5",
     "express": "^4.17.1",
-    "jsonwebtoken": "^8.5.1",
     "lodash": "^4.17.14"
   },
   "devDependencies": {
@@ -62,6 +61,7 @@
     "istanbul": "^0.4.5",
     "js-yaml": "^3.13.1",
     "jsdom": "^16.2.1",
+    "jsonwebtoken": "^8.5.1",
     "mocha": "^6.1.4",
     "mock-require": "^3.0.3",
     "mz": "^2.7.0",