Change git submodule protocol from ssh to https

Author: yusukebe

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "assets/revealjs"]
 	path = assets/revealjs
-	url = git@github.com:hakimel/reveal.js.git
+	url = https://github.com/hakimel/reveal.js.git
 [submodule "assets/socketio"]
 	path = assets/socketio
-	url = git@github.com:socketio/socket.io-client.git
+	url = https://github.com/socketio/socket.io-client.git