[201811][swss] advance swss sub-module head (#4037)

Submodule src/sonic-swss 49aff5da..e237e12d:
  > [aclorch] Validate that provided IN/OUT_PORTS are physical interfaces (#1156)
  > Teamd :: fix for cleaning up the teamd processes correctly on teamd docker stop (#1159)

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