Update course.yml

_data/course.yml

@@ -15,7 +15,7 @@ signups:
    image: https://pbs.twimg.com/profile_images/3602174360/daf674f339c583641f02dfeed3e6f3c6_400x400.jpeg
    twitter: schmidtphi
  - name: Carl
-   image: http://en.gravatar.com/userimage/57910423/9507615be0170dd86e97a27158c5f749.jpg
+   image: http://www.gravatar.com/avatar/8178cb8a778e8390a98e28b92d5ac764?s=400
    twitter: ender_78
  - name: Schuyler
    image: https://pbs.twimg.com/profile_images/1361718677/shoes_400x400.png