Fixes dwdyer#18

Make random.org requests in a static synchronized method, since random.org doesn't allow multiple requests in parallel.

core/src/java/main/org/uncommons/maths/random/RandomDotOrgSeedGenerator.java

@@ -98,7 +98,7 @@ public byte[] generateSeed(int length) throws SeedException
      * is greater than that permitted by random.org for a single request.
      * @throws IOException If there is a problem downloading the random bits.
      */
-    private void refreshCache(int requiredBytes) throws IOException
+    private static synchronized void refreshCache(int requiredBytes) throws IOException
     {
         int numberOfBytes = Math.max(requiredBytes, cache.length);
         numberOfBytes = Math.min(numberOfBytes, MAX_REQUEST_SIZE);