Add coveralls support by installing coveralls-php

Summary:
Coveralls is a web service to help to track the code coverage
over time, and ensure that all the new code is fully covered.
More Details: http://coveralls.io

Test Plan:
Run the unit tests and after that run coveralls. This should show you the code coverage
   $ vendor/bin/phpunit --colors -c tests/phpunit.xml --coverage-clover build/logs/clover.xml
   $ vendor/bin/coveralls -v

Reviewers: claus.due, #forgetit-dev, dkd-doan

Reviewed By: #forgetit-dev, dkd-doan

Differential Revision: https://phabricator.dkd.de/D399