Update ubuntu.ipxe

src/ubuntu.ipxe

@@ -15,6 +15,7 @@ item wily ${space} ${os} 15.10 Wily Werewolf
 item vivid ${space} ${os} 15.04 Vivid Vervet 
 item trusty ${space} ${os} 14.04 LTS Trusty Tahr
 item precise ${space} ${os} 12.04 LTS Precise Pangolin
+item --gap Pre-Releases
 item xenial ${space} ${os} 16.04 LTS Xenial Xerus Beta 
 item --gap Older Releases
 item older_release ${space} Set release codename...