fix "undefined method" error when assigning elastic IPs

Ironfan versions later than 4.6.1 were not working when an elastic IP was defined in the cluster file. The IP would not be assigned and ironfan would produce this error:

ERROR: undefined method `include?' for #<Ironfan::Dsl::Ec2:0x9466b90> (NoMethodError)
/var/lib/gems/1.9.1/gems/ironfan-4.10.4/lib/ironfan/provider/ec2/elastic_ip.rb:62:in `save!'