Update cloud_watch.rb

I see excon related errors when upgraded fog to version 0.18.0 from 0.15.0:

[excon][WARNING] Invalid Excon request keys: :host
/usr/lib/ruby/gems/1.8/gems/excon-0.28.0/lib/excon/connection.rb:217:in `request'
/usr/lib/ruby/gems/1.8/gems/fog-1.18.0/lib/fog/xml/sax_parser_connection.rb:36:in `request'
/usr/lib/ruby/gems/1.8/gems/fog-1.18.0/lib/fog/core/deprecated/connection.rb:18:in `request'
/usr/lib/ruby/gems/1.8/gems/fog-1.18.0/lib/fog/aws/cloud_watch.rb:153:in `_request'
/usr/lib/ruby/gems/1.8/gems/fog-1.18.0/lib/fog/aws/cloud_watch.rb:148:in `request'
/usr/lib/ruby/gems/1.8/gems/fog-1.18.0/lib/fog/aws/requests/cloud_watch/get_metric_statistics.rb:42:in `get_metric_statistics'

It can be fixed by the following change.