* fixes #257 - added $format arg to date histogram

* fixed typo

Example `['from' => 0, 'to' => 0]` is a valid aggregate range

(cherry picked from commit 8458ac9)

* fixed unit tests to support phpunit 6

* do not use sudo

* update elasticsearch in the travis env and use docker vm

* add java jdk8

* temporary remove elasticsearch scripting

* clean up travis file

* null cannot be used in count() functions anymore in 7.2

Fixed bug on nested and reverse nested aggregations. None of these aggregations requires the nested aggregations to be added. Test has also been updated. (#173)