[201811][utilities] advance sonic-utilities submodule

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

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