Update SlackMessageForwarder to use HTTP logo

Updating the call to for the logo to use https. We're trying to cut over to only support https on static.mparticle.com, so the logo wouldn't load here after that takes place.

examples/lambda/src/main/java/com/mparticle/sdk/samples/SlackMessageForwarder.java

@@ -118,7 +118,7 @@ private void postMessage(Account account, String messageText) throws IOException
         SlackPayload payload = new SlackPayload();
         payload.channel = channelName;
         payload.username = "LambdaForwarder";
-        payload.icon_url = "http://static.mparticle.com/public/mp-icon.png";
+        payload.icon_url = "https://static.mparticle.com/public/mp-icon.png";
         payload.text = messageText;
 
         try(CloseableHttpClient httpClient = HttpClients.createDefault()){