Pin yarl version to 0.18.* in setup.py

- We need to pin this dependency because version 1.0.0 of yarl
  does not expose `unquote` and if we do not pin it, the latest
  version >=1.*.* is downloaded and `aiohttp` expects it.
- This can also be fixed on `aiohttp`, but meanwhile :)

setup.py

@@ -83,6 +83,7 @@ install_requires = [
     'aiohttp~=2.0',
     'python-rapidjson-schema==0.1.1',
     'statsd==3.2.1',
+    'yarl>=0.11,<1.0',
 ]
 
 setup(