Fix error triggered in E_STRICT mode.

* Robo\Task\BaseComposerTask and Robo\Task\Shared\Executable define the
same property ($arguments) in the composition of Robo\Task\BaseComposerTask.
* Robo\Task\Codeception and Robo\Task\Shared\Executable define the same
property ($arguments) in the composition of Robo\Task\Codeception.
Merge pull request #25 from jadb/fix-strict-mode-error

Fix error triggered in E_STRICT mode.