use yojson >= 1.7.0, fix Yojson.json deprecation

ci: remove self before building

release 20190211

release 20190207

+ value_count aggregation

+ highlight in top_hits aggregation

open Devkit

sync tests

Merge pull request #8 from ahrefs/louis/mget-source-filter

add support for source filtering in mget requests

handle reverse_nested path, both default and explicit