The module should not have drush as a dependency

Problem/Motivation

Drush is a great tool but it is not required to make this module work. It shouldn't be declared as a dependency in the composer.json file.
If necessary for the tests, it should be declared as a dev dependency.
It could also be added to the suggests section to inform the persons requiring it that it has drush commands.

Proposed resolution

Remove drush from composer.json's dependencies.