CI: Fix screenshot diff

Turns out `-list metric` just lists the *available* metrics (AE, MAE,
NCC etc), not their actual values; it also doesn't create the actual
diff images.

circle.yml

diff --git a/circle.yml b/circle.yml
index 8a0b308..67e81cb 100644
--- a/circle.yml
+++ b/circle.yml
@@ -212,8 +212,8 @@
         # if evergreen browser, browser version of previous screenshot may not match,
         # so replace previous browser version with glob
         baseline=baseline/"$(sh -c "ls $(echo $file | sed 's/[^_]*_(evergreen)/*/')")"
-        echo "Diff metrics for $file:"
-        compare -list metric $baseline $file ${file/%.png/_DIFF.png}
+        echo "Number of different pixels from baseline in $file:"
+        compare -metric AE $baseline $file ${file/%.png/_DIFF.png}
       done
   post:
     - killall --wait sc; true  # wait for Sauce Connect to close the tunnel; ignore errors since it's just cleanup