Require local phpunit through composer and bump phpunit version

* Uses PHPUnit 6.5 (requires PHP 7.0+, but we got rid with NC 14 (see nextcloud/server#7368)
* Namespaces are rewritten
* If composer isn't installed, it installs it (taken from nextcloud/news)

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Remove PHPUnit Download in CI

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Properly handle phpunit on travis for 5.6

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Remove trouble composer.lock

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

Don't install composer deps twice on CI

Signed-off-by: Thomas Citharel <tcit@tcit.fr>