Updated profile identifier JSON key (#163)

In cupertino `1.2.2` (latest) I'm seeing a similar error to #163 in a slightly different location when trying to download profiles. It seems that Apple's JSON has been updated once again, and this tweak appears to fix the issue for me.

```bash
/Library/Ruby/Gems/2.0.0/gems/cupertino-1.2.2/lib/cupertino/provisioning_portal/agent.rb:218:in `block in list_profiles': undefined method `[]' for nil:NilClass (NoMethodError)
	from /Library/Ruby/Gems/2.0.0/gems/cupertino-1.2.2/lib/cupertino/provisioning_portal/agent.rb:210:in `each'
	from /Library/Ruby/Gems/2.0.0/gems/cupertino-1.2.2/lib/cupertino/provisioning_portal/agent.rb:210:in `list_profiles'
	from /Library/Ruby/Gems/2.0.0/gems/cupertino-1.2.2/lib/cupertino/provisioning_portal/commands/profiles.rb:65:in `block (3 levels) in <top (required)>'
	from /Library/Ruby/Gems/2.0.0/gems/cupertino-1.2.2/lib/cupertino/provisioning_portal/helpers.rb:58:in `try'
	from /Library/Ruby/Gems/2.0.0/gems/cupertino-1.2.2/lib/cupertino/provisioning_portal/commands/profiles.rb:65:in `block (2 levels) in <top (required)>'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/command.rb:180:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/command.rb:180:in `call'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/command.rb:155:in `run'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/runner.rb:421:in `run_active_command'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/runner.rb:81:in `run!'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/delegates.rb:8:in `run!'
	from /Library/Ruby/Gems/2.0.0/gems/commander-4.2.1/lib/commander/import.rb:10:in `block in <top (required)>'
```