CI: Suppress visual diff errors

Especially "image widths or heights differ" which happens a lot:
https://github.com/mathquill/mathquill/issues/649#issuecomment-228233818

Also https://circleci.com/gh/mathquill/mathquill/365 if you go and look
at the output of the `...compare -metric...` line, you can see that an
"image widths or heights differ" warning was logged (but then a
subsequent diff succeeded so Circle thinks the command worked overall).

circle.yml

diff --git a/circle.yml b/circle.yml
index be4994b..3cdc574 100644
--- a/circle.yml
+++ b/circle.yml
@@ -216,5 +216,6 @@
         echo "Number of different pixels from baseline in $file:"
         compare -metric AE $baseline $file ${file/%.png/_DIFF.png}
       done
+      true  # ignore errors like "image widths or heights differ"
   post:
     - killall --wait sc; true  # wait for Sauce Connect to close the tunnel; ignore errors since it's just cleanup