Add CompositeAggregation#missingBucket

[exoego]

Addresses one of breking change in ES v6.0.

This PR adds CompositeAggregation#missingBucket.

[sudo-suhas]

Hey @exoego, thanks for the PR. Apart from the review comments, could you also add a test alongside and similar to 👇

elastic-builder/test/aggregations-test/composite-agg-values-sources-test/terms-values-source.test.js

Line 38 in 39c0f90

test(setsOption, 'missing', { param: 42 });

Thanks!

[sudo-suhas]

Change to Specifies whether to include documents without a value for a given source in the response. Defaults to false (not included).. Could you also wrap the comment at 80 characters?

[exoego]

Addressed in 96fc517

[sudo-suhas]

Suggested change

	* [Elastticsearch v6.0](https://www.elastic.co/guide/en/elasticsearch/reference/6.8/breaking-changes-6.0.html#_literal_missing_literal_is_deprecated_in_the_literal_composite_literal_aggregation),
	* [Elasticsearch v6](https://www.elastic.co/guide/en/elasticsearch/reference/6.8/breaking-changes-6.0.html#_literal_missing_literal_is_deprecated_in_the_literal_composite_literal_aggregation),

[exoego]

Addressed in 96fc517

[sudo-suhas]

Could you add this note here as well 👇

elastic-builder/src/aggregations/bucket-aggregations/composite-agg-values-sources/values-source-base.js

Lines 96 to 106 in 39c0f90

	/**
	* Missing specifies the value to use when the source finds a missing value
	* in a document.
	*
	* @param {string} value
	* @returns {ValuesSourceBase} returns `this` so that calls can be chained
	*/
	missing(value) {
	this._opts.missing = value;
	return this;
	}

[exoego]

Addressed in 96fc517

[sudo-suhas]

Thank you @exoego! Changes have been released in elastic-builder@2.5.0 🎉