fix missing require

We've run into the following:

```

uninitialized constant MCollective::RPC::Stats::StringIO (NameError)
	from /usr/lib/ruby/1.8/mcollective/rpc/stats.rb:136:in `text_for_aggregates'  <----
	from /usr/lib/ruby/1.8/mcollective/rpc/stats.rb:192:in `report'
	from /usr/lib/ruby/1.8/mcollective/rpc.rb:137:in `printrpcstats'
	from /usr/share/mcollective/plugins/mcollective/application/puppet.rb:280:in `disable_command'
	from /usr/share/mcollective/plugins/mcollective/application/puppet.rb:322:in `send'
	from /usr/share/mcollective/plugins/mcollective/application/puppet.rb:322:in `main'
	from /usr/lib/ruby/1.8/mcollective/application.rb:285:in `run'
	from /usr/lib/ruby/1.8/mcollective/applications.rb:23:in `run'
	from /usr/bin/mco:20
```

Adding this require clears up the error.