Add Heroku to sponsor section

Author: calebhearth

index.html

@@ -301,6 +301,13 @@ <h2>Meet the sponsors</h2>
           <img alt="One More Cloud logo" src="/assets/images/one-more-cloud.png">
         </a>
       </li>
+      
+      
+      <li class="sponsors__list-item">
+        <a href="https://heroku.com/">
+          <img alt="Heroku logo" src="/assets/images/heroku.svg">
+        </a>
+      </li>
     </ul>
 
     <div class="sponsor">