airflow 2 support

[KostyaEsmukov]

This PR adds support for airflow 2, and:

• Drops support for airflow 1 (to avoid complicating the tests, because airflow.operators.dummy is not importable in v1, and airflow.operators.dummy_operator is not importable in v2),
• Drops support for python 2.7 (airflow 2 doesn't support it),
• Drops .travis.yml (I guess it doesn't work anymore).

I have tested this in a "hello world"-like environment and it seems to work fine.

@kxepal feel free to make any alterations on top if you want to.

[KostyaEsmukov]

I've been running this for a week on 2.1.3 -- haven't found any issues so far.