Fixed docs

Author: boxed

docs/_templates/ghbuttons.html

@@ -1,4 +1,4 @@
 <h3>Github</h3>
-<iframe src="http://ghbtns.com/github-btn.html?user=boxed&repo=mutmut&type=watch&count=true&size=large"
+<iframe src="https://ghbtns.com/github-btn.html?user=boxed&repo=mutmut&type=watch&count=true&size=large"
         frameborder="0" scrolling="0" width="170" height="30" allowtransparency="true"></iframe>
 <br><br>