AssociatePublicIpAddress support for goamz/packer

Provides required underlying AssociatePublicIpAddress support for goamz, so that packer can use it.

Needed to bump the EC2 API version here, I don't suspect any other breakage looking at their release notes though.

Ignore my testing/revert testing commits, files changed shows the relevant stuff at least.

Relates to https://github.com/mitchellh/packer/issues/578