Adds symfony/process to dependencies to avoid issues on Windows environment

[jeremyb]

Hi,

I tried to use this library on a Windows environment and encountered an issue with process execution. I got a "0" as exit status but nothing in stdout... I tried to debug and finally realised that a different behaviour was possible by installing symfony/process package. And it worked well with it.

So, instead of try to deal with Windows environment in the embedded Process class, why not add it to the dependencies? (they already have some code for that: https://github.com/symfony/Process/blob/master/Process.php#L277)

I also think that it's not a good idea to provide two implementations of process execution with a silent configuration (by silent I mean if (class_exists('Symfony\Component\Process\Process')) {}), it doesn't ease the maintenance.

[jeremyb]

We discussed with @docteurklein that it could be possible to add an Adapter pattern for process execution, a kind of CommandExecutor interface, with an implementation through symfony/process. But for now the executeCommand method returns an array with $process->getExitCode(), $process->getOutput(), $process->getErrorOutput() so a Process is required and probably needs some refactoring IMO.

[docteurklein]

LGTM

[akovalyov]

NULL should be lowercased according to PSR-2

[jeremyb]

Done @akovalyov :)

[akovalyov]

LGTM