CI: Ensure concatenating screenshot pieces in right order

On some systems, `ls ...*...` lists them in random order; and if there
are ever >9 pieces, lexicographic sort will be wrong, must sort in
numeric order.

circle.yml

diff --git a/circle.yml b/circle.yml
index 13f03b1..62ffec7 100644
--- a/circle.yml
+++ b/circle.yml
@@ -178,7 +178,7 @@
     # Stitch together images
     - |-
       for img in $(ls $CIRCLE_ARTIFACTS/imgs/pieces/); do
-        convert $CIRCLE_ARTIFACTS/imgs/pieces/$img/*.png -append $CIRCLE_ARTIFACTS/imgs/$img.png
+        convert $(ls -1 $CIRCLE_ARTIFACTS/imgs/pieces/$img/*.png | sort -n) -append $CIRCLE_ARTIFACTS/imgs/$img.png
       done
 
     # Download the latest mathquill artifacts from master.