Relax assertWarns to fix Windows py27 build
Relax assertWarns to fix Windows py27 build

shoyer/xray,jcmgray/xarray,markelg/xray,xray/xray,pydata/xarray,shoyer/xarray,pydata/xarray,jhamman/xarray,shoyer/xarray,chunweiyuan/xarray,jhamman/xray,jhamman/xarray,rabernat/xray,markelg/xray,rabernat/xray,markelg/xray,jhamman/xarray,pydata/xarray