Temporarily pin yarl while aiohttp is broken

Aiohttp had an open requirement specification on yarl which has
released a 1.0 that is backwards incompatible.  Pin to <1.0
until https://github.com/aio-libs/aiohttp/issues/2662 is fixed.

Change-Id: I4e750900501ed92bdbb616f5664f7e8ab7fa99c3
diff --git a/requirements.txt b/requirements.txt
index 39a2b02..193c64e 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -25,5 +25,6 @@
 cachecontrol
 pyjwt
 iso8601
+yarl>=0.11,<1.0
 aiohttp
 uvloop;python_version>='3.5'