cfn2py dies while processing SecurityGroupEgress

It would seem I have the best CF template ever for testing `cfn2py`.

The script is now returning this error:

```
NatSecurityGroup = t.add_resource(SecurityGroup(
"NatSecurityGroup",
VpcId=Ref(MyVpc),
GroupDescription="NAT VPC security group",
Traceback (most recent call last):
File "./cfn2py", line 229, in
globals()["do_" + s.lower()](d)
File "./cfn2py", line 128, in do_resources
print ' %s=%s,' % (pk, output_value(pv))
File "./cfn2py", line 178, in output_value
return "[" + ", ".join(out) + "]"
TypeError: sequence item 0: expected string, NoneType found
```

with the following as the input template snipped

``` json
"NatSecurityGroup" : {
"Type" : "AWS::EC2::SecurityGroup",
"Properties" : {
"GroupDescription" : "NAT VPC security group",
"VpcId" : {
"Ref" : "MyVpc"
},
"SecurityGroupEgress" : [ {
"IpProtocol" : "-1",
"CidrIp" : "0.0.0.0/0"
} ]
}
},
```

Please let me know if you need any more information.