Allow using only hostname with ?superdevmode

Change-Id: Ib88062d13fc3a22970b526a7fc56a22ba860637f

client/src/com/vaadin/client/SuperDevMode.java

         if (serverUrl == null || "".equals(serverUrl)) {
             serverUrl = "http://localhost:9876/";
         } else {
-            serverUrl = "http://" + serverUrl + "/";
+            if (serverUrl.contains(":")) {
+                serverUrl = "http://" + serverUrl + "/";
+            } else {
+                serverUrl = "http://" + serverUrl + ":9876/";
+            }
         }
 
         if (hasSession(SKIP_RECOMPILE)) {