Update raven to 6.8.0

[pyup-bot]

This PR updates raven from 6.7.0 to 6.8.0.

Changelog

6.8.0

------------------
* [Core] Fixed DSNs without secrets not sending events.
* [Core] Added lazy import for pkg_resources
* [Core] Added NamedTuple Serializer
* [Sanic] Fixed sanic integration dependencies
* [Django] Fixed sql hook bug

Links

• PyPI: https://pypi.org/project/raven
• Changelog: https://pyup.io/changelogs/raven/
• Repo: https://github.com/getsentry/raven-python

[pyup-bot]

Closing this in favor of #14