Update to ES 6

[josemlp91]

@myarik We are using the package "django-rest-elasticsearch" and it is required update our elasticsearch cluster to 6.7 version, here the minor changes applied:

• Update elasticsearch-dsl
• Update test with compatibitity to new elasticsearch-dsl api.
• Minor change in test_filters.test_filter_search assert.
• Update README example.
• Create Dockerfile and docker-compose, usefull to run the test with diferent configurations.

I wish this change are welcome, and usefull to other developer, in the future I want continue work in other issues related.

Best Regard.

[myarik]

@josemlp91 Thanks for your contribution