Fix method visibility (#12)

The method AvroIODatumReader::skip_data calls the
AvroIOBinaryDecoder::skip_long method, which is protected. The result is
a fatal error.

Change the AvroIOBinaryDecoder::skip_long method visibility from
protected to public as all the others skip_* methods.

lib/avro/datum.php

@@ -1015,7 +1015,7 @@ public function skip_boolean() { return $this->skip(1); }
 
   public function skip_int() { return $this->skip_long(); }
 
-  protected function skip_long()
+  public function skip_long()
   {
     $b = ord($this->next_byte());
     while ('' == $b || 0 != ($b & 0x80))