updates for spdep 1.2-8

[jeffeaton]

spdep 1.2-8 added warnings to require some arguments in mat2listw().

converged: relative convergence (4)
Warning messages:
1: In spdep::mat2listw(abs(Q), style = "M") :
  style is M (missing); style should be set to a valid value
2: In spdep::mat2listw(abs(Q), style = "M") :
  no-neighbour observations found, set zero.policy to TRUE;
this warning will soon become an error