CI: <FOR TESTING ONLY> Use ci.cleanup instead of master

Because we changed screenshot filename format so we can't find the
matching baseline one

circle.yml

diff --git a/circle.yml b/circle.yml
index c9ef97d..7f1fe50 100644
--- a/circle.yml
+++ b/circle.yml
@@ -183,7 +183,8 @@
 
     # Download the latest screenshots from master.
     - |-
-      artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=master)"
+      artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=ci.cleanup)"
+      #when done testing, restore: artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=master)"
       echo
       echo '/latest/artifacts?branch=master:'
       echo