Fix case for Sanic dependency

On PyPI it has a capital letter: https://pypi.python.org/pypi/Sanic

I think this is why the list of "Requires Distributions" on https://pypi.python.org/pypi/datasette links to all of the other dependencies but does not link to Sanic.

setup.py

@@ -14,7 +14,7 @@ setup(
     install_requires=[
         'click==6.7',
         'click-default-group==1.2',
-        'sanic==0.6.0',
+        'Sanic==0.6.0',
         'Jinja2==2.10',
         'sanic-jinja2==0.5.5',
         'hupper==1.0',