Force https download for avatars. Leverage http2 from opencollective

Author: Munter

docs/_includes/backers.md

@@ -3,4 +3,4 @@
 Find Mocha helpful?  Become a [backer](https://opencollective.com/mochajs#support) and support Mocha with a monthly donation. 
 
 {: id="_backers" class="image-list" }
-{% for i in (0..29) %}[![](//opencollective.com/mochajs/backer/{{ i }}/avatar.jpg)](https://opencollective.com/mochajs/backer/{{ i }}/website){: target="_blank" rel="noopener"}{% endfor %}
+{% for i in (0..29) %}[![](https://opencollective.com/mochajs/backer/{{ i }}/avatar.jpg)](https://opencollective.com/mochajs/backer/{{ i }}/website){: target="_blank" rel="noopener"}{% endfor %}