Breadcrumbs type error

We're having an issue with v0.3.2 where `popleft` is called on a list type:

```
File "/srv/frontend/project/lib/python3.5/site-packages/sentry_sdk/hub.py" in add_breadcrumb
  209.             scope._breadcrumbs.popleft()

Exception Type: AttributeError at /
Exception Value: 'list' object has no attribute 'popleft'
```