[MINOR] fix contributors pictures (#151)

Author: Baunsgaard

_src/community.html

@@ -73,7 +73,7 @@ <h2>SystemDS Project Committers</h2><br>
       <div class="profile">
         <a href="http://github.com/{{ member.githubId }}">
           <img class="profile--img"
-            src="{% unless c.avatar %}http://github.com/{{ member.githubId }}.png{% else %}{{ member.avatar }}{% endunless %}">
+            src="{% unless c.avatar %}https://avatars.githubusercontent.com/{{ member.githubId }}{% else %}{{ member.avatar }}{% endunless %}">
         </a>
         <div class="profile--info">
           <a class="profile--name" href="http://github.com/{{ member.githubId }}">{{member.name}}</a>