[201811][utilities] advance sonic-utilities submodule (#2792)

Submodule src/sonic-utilities 6aee909..79a0185:
  > [fast/warm reboot] add some sanity check before warm reboot (#510)
  > In sync with our latest change, where we default failthrough to be False. (#507)
  > [generate_dump] system dump improvements (#503)

Signed-off-by: Ying Xie <ying.xie@microsoft.com>