Filter out files have `PREV` in them

When we downloaded all of the files we were getting all of the assests
that ended with `png`. This included the previous `PREV` artifacts.

circle.yml

diff --git a/circle.yml b/circle.yml
index c29f2c6..2cb24c0 100644
--- a/circle.yml
+++ b/circle.yml
@@ -145,6 +145,7 @@
       curl $(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts \
               | json -a url pretty_path -d '\n\t' \
               | grep '\.png$' \
+              | grep -V 'PREV' \
               | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/PREV_:")
       cd $CIRCLE_ARTIFACTS/imgs/