phpunit: use executable in vendor/bin

Fixes:
$ phpunit --verbose
> PHPUnit 6.2.3 by Sebastian Bergmann and contributors.
> Runtime:       PHP 7.0.21 with Xdebug 2.5.5
> [...]
> No tests executed!

.travis.yml

@@ -11,4 +11,4 @@ install:
 - composer global require "fxp/composer-asset-plugin:^1.3.1"
 - composer install
 
-script: phpunit --verbose
+script: ./vendor/bin/phpunit --verbose