Change timeboard.update to use PUT, per DataDog API

lib/api/timeboard.js

@@ -144,7 +144,7 @@ function update(dashId, title, description, graphs, templateVariables, callback)
         params.body.template_variables = templateVariables;
     }
 
-    client.request("POST", util.format("/dash/%s", dashId), params, callback);
+    client.request("PUT", util.format("/dash/%s", dashId), params, callback);
 }
 
 /*section: timeboard