corrected URI reference so it uses the standard lib

First off, thanks for writing this rubygem, it's the best one out there in my opinion... The others just simply suck, but that's partially due to the enormous size of Zendesk's API.

Some context:
- Using Ruby 2.1.1 via RVM
- Mac OS X 10.9.4
- Using ~/.zendesk2 (btw- thank you for this lovely design. It is wonderful)

```irb
2.1.1 :001 > require 'zendesk2'
 => true
2.1.1 :002 > client = Zendesk2::Client.new
NameError: uninitialized constant Zendesk2::Client::Real::URI
	from /Users/jbarnett/.rvm/gems/ruby-2.1.1/gems/zendesk2-0.5.3/lib/zendesk2/client.rb:159:in `initialize'
	from /Users/jbarnett/.rvm/gems/ruby-2.1.1/gems/cistern-0.9.1/lib/cistern/service.rb:181:in `new'
	from /Users/jbarnett/.rvm/gems/ruby-2.1.1/gems/cistern-0.9.1/lib/cistern/service.rb:181:in `new'
	from (irb):2
	from /Users/jbarnett/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'
```

I fixed this issue with these code changes.