title: Dates not sorted for date histogram in elasticsearch
body: <p>Im firing a query, only with date_histogram aggregation like below</p>
  
  <pre><code>{
    "aggs": {
      "mentionsAnalytics": {
        "date_histogram": {
          "field": "created_at",
          "interval": "day"
        }
      }
    }
  </code></pre>
  
  <p>Now the response Im getting is not sorted according to date. Following is my response</p>
  
  <pre><code>"aggregations": {
  "mentionsAnalytics": {
  "buckets": [
  {
  "key_as_string": "2014-10-17T00:00:00.000Z",
  "key": 1413504000000,
  "doc_count": 2
  }
  ,
  {
  "key_as_string": "2015-09-07T00:00:00.000Z",
  "key": 1441584000000,
  "doc_count": 2
  }
  ,
  {
  "key_as_string": "2015-09-29T00:00:00.000Z",
  "key": 1443484800000,
  "doc_count": 2
  }
  ,
  {
  "key_as_string": "2015-11-09T00:00:00.000Z",
  "key": 1447027200000,
  "doc_count": 4
  }
  ]
  }
  }
  </code></pre>
  
  <p>As you can see, the dates are occuring in random order. Can I make it sorted,by modifying any parameters inside the date_histogram aggregation. I dont prefer to give a sort query, as a seperate query. Is that possible?</p>