deploy function app HTTP 409 error

**Describe the bug**
When trying to upload a function app the zip upload returns a 409 and fails.

**To Reproduce**
I'm not sure how to reproduce outside our specific function apps. We've previously used the `func deploy` command to deploy and are now moving to using the `az functionapp deployment source config-zip {...}` command. Upon creating a new function app the deploy worked successfully.

**Expected behavior**
I would expect it to deploy.

**Environment summary**

Travis CI
Ubuntu 14.04 - trusty
az (latest installed from apt)
bash

**Additional context**
```
WARNING: Starting zip deployment
DEBUG: urllib3.connectionpool : Starting new HTTPS connection (1): permitbook-service-dev-proxy.scm.azurewebsites.net:443
DEBUG: urllib3.connectionpool : https://permitbook-service-dev-proxy.scm.azurewebsites.net:443 "POST /api/zipdeploy?isAsync=true HTTP/1.1" 409 121
DEBUG: urllib3.connectionpool : Starting new HTTPS connection (1): permitbook-service-dev-proxy.scm.azurewebsites.net:443
DEBUG: urllib3.connectionpool : https://permitbook-service-dev-proxy.scm.azurewebsites.net:443 "GET /api/deployments/latest HTTP/1.1" 200 653
ERROR: Zip deployment failed. {'id': '6121545938061494', 'status': 3, 'status_text': '', 'author_email': '', 'author': 'Alex Moon', 'deployer': 'VSTS', 'message':
```

https://httpstatuses.com/409