CI: Diff screenshots against master not any latest

The latest build could be from any random broken branch, when actually
we only want to diff the screenshot of this branch against the latest
screenshot on master.

circle.yml

diff --git a/circle.yml b/circle.yml
index 0012906..ce28761 100644
--- a/circle.yml
+++ b/circle.yml
@@ -183,11 +183,11 @@
     # the previous test run plus whichever new images were generated.
     - rm -R -- $CIRCLE_ARTIFACTS/imgs/*/
 
-    # Download the latest mathquill artifacts.
+    # Download the latest mathquill artifacts from master.
     - |-
-      artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts)"
+      artifacts_json="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts?branch=master)"
       echo
-      echo '/latest/artifacts:'
+      echo '/latest/artifacts?branch=master:'
       echo
       echo "$artifacts_json"
       echo