Fix bug that makes mastermind.s unplayable

This is a regression not present in v1.5.

It seems to have been introduced by 43dd8a7.
Specifically double buffering System.out seems to hold things up. Simply
flushing constantly seems to fix it.

src/rars/util/SystemIO.java

@@ -142,6 +142,7 @@ public static void printString(String string) {
         if (Globals.getGui() == null) {
             try {
                 SystemIO.getOutputWriter().write(string);
+                SystemIO.getOutputWriter().flush();
             } catch (IOException e){
             }
         } else {