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.

Author: youcandanch

setup.py

@@ -61,7 +61,10 @@
 
 # If it's Python 3.5+, add Sanic packages.
 if sys.version_info >= (3, 5):
-    sanic_requires = ['sanic>=0.7.0', ]
+    sanic_requires = [
+        'blinker>=1.1',
+        'sanic>=0.7.0', 
+    ]
     sanic_tests_requires = ['aiohttp', ]
 
 tests_require = [