Add #626 coverage generation from phpunit

[ttibensky]

we need to upload the artifacts somewhere regarding to https://docs.travis-ci.com/user/uploading-artifacts/

or we can use alternative service instead of travis, for example https://circleci.com/ which is free if we use a single parallel container + you can upload and review artifacts there

we could also probably use following code analysis tools all of which can be configured to fail the travis build:

• phpmd ./vendor/bin/phpmd ./src html codesize,unusedcode --reportfile ./coverage/phpmd.html - generates html report
• phpmd ./vendor/bin/phpcs src --generator=HTML > ./coverage/phpcs.html - generates html report
• phpstan vendor/bin/phpstan analyse src tests --memory-limit=-1 --error-format=table prints issues which would cause a fatal error
• other tools can be viewed here https://web-techno.net/code-quality-check-tools-php/