add conditional

sorta supports Chef 10.x in that it won't blow up, but COOK-4213 will
not be fixed for Chef 10.x by design.
Merge branch 'lcg/CHEF-4213' of github.com:opscode-cookbooks/aws into opscode-cookbooks-lcg/CHEF-4213

COOK-4213: adjusting so that it doesn't blow up Chef 10.  Author: Lamont Granquist.
Merge branch 'opscode-cookbooks-lcg/CHEF-4213'