Fix aggregations when running ES 1.4.0.Beta1

And also add support for "doc_count_error_upper_bound" that was introduced
in ES 1.4.0.Beta1.

Closes #1025

Author: gmarz

src/Nest/Domain/Aggregations/Bucket.cs

@@ -10,6 +10,7 @@ public class Bucket<TBucketItem> : BucketAggregationBase
 	public class Bucket : IAggregation
 	{
 		public IEnumerable<IAggregation> Items { get; set; }
+		public long? DocCountErrorUpperBound { get; set; }
 	}
 
 	public class BucketWithDocCount<TBucketItem> : BucketAggregationBase, IBucketWithCountAggregation

src/Nest/Resolvers/Converters/Aggregations/AggregationConverter.cs

@@ -45,6 +45,7 @@ private IAggregation ReadAggregation(JsonReader reader, JsonSerializer serialize
 				case "value":
 					return GetValueMetricOrAggregation(reader, serializer);
 				case "buckets":
+				case "doc_count_error_upper_bound":
 					return GetBucketAggregation(reader, serializer);
 				case "key":
 					return GetKeyedBucketItem(reader, serializer);
@@ -292,6 +293,14 @@ private IDictionary<string, IAggregation> GetNestedAggregations(JsonReader reade
 		private IAggregation GetBucketAggregation(JsonReader reader, JsonSerializer serializer)
 		{
 			var bucket = new Bucket();
+			var property = reader.Value as string;
+			if (property == "doc_count_error_upper_bound")
+			{
+				reader.Read();
+				bucket.DocCountErrorUpperBound = reader.Value as long?;
+				reader.Read();
+			}
+
 			var aggregations = new List<IAggregation>();
 			reader.Read();
 			if (reader.TokenType != JsonToken.StartArray)