(maint) move statsd-ruby require into Vmpooler::Statsd class

We've managed to move mentions of this out of the calling code, so let's
move the require.

lib/vmpooler.rb

@@ -7,7 +7,6 @@ module Vmpooler
   require 'rbvmomi'
   require 'redis'
   require 'sinatra/base'
-  require "statsd-ruby"
   require 'time'
   require 'timeout'
   require 'yaml'

lib/vmpooler/statsd.rb

@@ -1,4 +1,5 @@
 require 'rubygems' unless defined?(Gem)
+require 'statsd-ruby'
 
 module Vmpooler
   class Statsd