Add blinker to Sanic dependencies.
Add blinker to Sanic dependencies.

Pulling 6.7.0 and trying to do, `from raven.contrib.sanic import Sentry`, I get the following traceback:

```

Traceback (most recent call last):
  File "application.py", line 1, in <module>
    from raven.contrib.sanic import Sentry
  File "/usr/local/lib/python3.6/site-packages/raven/contrib/sanic.py", line 13, in <module>
    import blinker
ModuleNotFoundError: No module named 'blinker'
```

That's my bad. This should resolve that.
getsentry/raven-python,getsentry/raven-python,getsentry/raven-python