[swss]: update swss submodule

* a1b6fa3 2018-04-16 | [vlanmgr]: Fix the incorrect ip link del command for vlan remove (#474) (HEAD, origin/201803) [Jipan Yang]
* 66287cf 2018-05-18 | fix SAI_BUFFER_PROFILE_ATTR_SHARED_DYNAMIC_TH value type mismatch bug (#502) [yangbashuang]
* 43365f3 2018-05-14 | [crmorch]: neighbor used counter increased twice (#472) (#501) [MaxPolovyi]

Signed-off-by: Guohan Lu <gulv@microsoft.com>