Load gravatars over protocol-relative links.

Fixes mixed content warnings on https://consul.io/community.html

website/source/community.html.erb

@@ -42,7 +42,7 @@ description: |-
 			</p>
 			<div class="people">
 				<div class="person">
-					<img class="pull-left" src="http://www.gravatar.com/avatar/11ba9630c9136eef9a70d26473d355d5.png?s=125">
+					<img class="pull-left" src="//www.gravatar.com/avatar/11ba9630c9136eef9a70d26473d355d5.png?s=125">
 					<div class="bio">
 						<h3>Armon Dadgar (<a href="https://github.com/armon">@armon</a>)</h3>
 						<p>
@@ -58,7 +58,7 @@ description: |-
 				<div class="clearfix"></div>
 
 				<div class="person">
-					<img class="pull-left" src="http://www.gravatar.com/avatar/54079122b67de9677c1f93933ce8b63a.png?s=125">
+					<img class="pull-left" src="//www.gravatar.com/avatar/54079122b67de9677c1f93933ce8b63a.png?s=125">
 					<div class="bio">
 						<h3>Mitchell Hashimoto (<a href="https://github.com/mitchellh">@mitchellh</a>)</h3>
 						<p>
@@ -76,7 +76,7 @@ description: |-
 				<div class="clearfix"></div>
 
 				<div class="person">
-					<img class="pull-left" src="http://www.gravatar.com/avatar/2acc31dd6370a54b18f6755cd0710ce6.png?s=125">
+					<img class="pull-left" src="//www.gravatar.com/avatar/2acc31dd6370a54b18f6755cd0710ce6.png?s=125">
 					<div class="bio">
 						<h3>Jack Pearkes (<a href="https://github.com/pearkes">@pearkes</a>)</h3>
 						<p>