update license information

[valeriocos]

Most files in Graal come with a old copyright date (e.g., # Copyright (C) 2015-2018 Bitergia -> https://github.com/chaoss/grimoirelab-graal/blob/master/tests/base_analyzer.py#L4). The goal of this issue is to check each file included in Graal and update the copyright dates where needed.

[inishchith]

@valeriocos I'd like to work on this issue in case this is not being worked on 😄

[valeriocos]

perfect! thanks!