* phpunit: use dedicate `Tests` namespace for tests
* tests: replace phpunit with orchestra/testbench
* phpunit: ignore result cache file
* phpunit: adapt test for newer version
* tests: add base testcase class with a helper to run/assert commands
* tests: add test for ide-helper:eloquent
* travis: add global default variables
* travis: disable xdebug, speeds up running tests
* travis: convert php versions to build matrix for environment vars
* travis: only run phpcs/phpunit when the env vars signal it
However phpcs only runs with one test combination from the matrix,
not necessary to execute it every time.
