replace group-name filter with instance.group-name

Use instance.group-name instead of group-name everywhere when filtering
instances. Turns out I was wrong about instance.group-name being a
VPC-only filter. This filter appears to work for both default VPC and
non-VPC instances and is a much better fix for #265 than the previous
commit. Thanks to @syoyo for suggesting the proper fix all along.

closes gh-265
closes gh-270