CI: Use master not ci.cleanup as baseline screenshots

Revert of a3484e4c135573f0158ca0e89084af1e550a2a6a, which was for
testing only.

circle.yml

diff --git a/circle.yml b/circle.yml
index e05aea8..331b639 100644
--- a/circle.yml
+++ b/circle.yml
@@ -187,8 +187,7 @@
       echo '2. Download the latest screenshots from master'
       echo
 
-      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)"
+      artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=master)"
       echo
       echo '/latest/artifacts?branch=master:'
       echo