How to handle Drush extensions

[taz77]

I have a Drupal module that relies on Composer Manager module and the Composer extension for Drush.

Everything goes well till after the Composer extension is installed for Drush. I need to find how I can inject a clear cache on drush after the extension loads.

Here is the travis log output

Download and install the Drush Composer extension? (y/n): y
Project composer (8.x-1.x-dev) downloaded to [success]
/home/travis/.drush/composer.
Project composer contains 0 modules: .
The drush command 'composer' could not be found. [error]