Fix ganglia input to work again.

Tested with:
  % ruby -rgmetric -e 'Ganglia::GMetric.send("127.0.0.1", 8649, { :name => "pageviews", :units => "req/min", :type => "uint8", :value => 7000, :tmax => 60, :dmax => 300, :group => "test" })'

Result:
  % bin/logstash agent -e 'input { ganglia { } }'
  {
    "@timestamp" => "2013-12-03T17:46:04.327Z",
    "@version"   => "1",
    "log_host"   => "",
    "dmax"       => 300,
    "tmax"       => 60,
    "slope"      => "both",
    "type"       => "uint8",
    "units"      => "req/min",
    "host"       => "127.0.0.1"
  }