Chokes on VPC custom protocol rules

I get a failure in ip_permissions.rb:45 when processing a permission like this:

```ruby
{"groups"=>[{"userId"=>"12345", "groupId"=>"sg-12345"}], "ipRanges"=>[], "ipProtocol"=>"-1"}
```
This was a VPC security group created by selecting "Custom protocol rule" from the dropdown, -1 for the protocol and sg-12345 for the group id.