Update link to roulette status images

We are using PNG instead of SVGs for compatibility with email
notifications.

danger/roulette/Dangerfile

@@ -13,8 +13,8 @@ MARKDOWN
 POST_TABLE_MESSAGE = <<MARKDOWN
 
 **Please check reviewer's status!**
-- ![available](https://gitlab-org.gitlab.io/gitlab-roulette/status-svg/_success.svg) Reviewer is available!
-- ![unavailable](https://gitlab-org.gitlab.io/gitlab-roulette/status-svg/_alert.svg) Reviewer is unavailable!
+- ![available](https://gitlab-org.gitlab.io/gitlab-roulette/status-png/_success.png){width=12} Reviewer is available!
+- ![unavailable](https://gitlab-org.gitlab.io/gitlab-roulette/status-png/_alert.png){width=12} Reviewer is unavailable!
 
 Please refer to [documentation page](https://docs.gitlab.com/ee/development/code_review.html#reviewer-roulette)
 for guidance on how you can benefit from the Reviewer Roulette, or use the