Enable SIGINT handler

If I don't, I and CTRL+C during the download step, there is LOT of error:

```
Warning: curl_exec(): Could not call the CURLOPT_WRITEFUNCTION in /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Adapter/Curl/Curl
Adapter.php on line 86

Call Stack:
    0.0004     122832   1. {main}() /home/greg/dev/labs/symfony-installer/symfony:0
    0.0159     909508   2. Symfony\Component\Console\Application->run() /home/greg/dev/labs/symfony-installer/symfony:16
    0.0200    1123116   3. Symfony\Component\Console\Application->doRun() /home/greg/dev/labs/symfony-installer/vendor/symfony/console/Symfony/Compone
nt/Console/Application.php:124
    0.0201    1123560   4. Symfony\Component\Console\Application->doRunCommand() /home/greg/dev/labs/symfony-installer/vendor/symfony/console/Symfony/
Component/Console/Application.php:193
    0.0202    1123796   5. Symfony\Component\Console\Command\Command->run() /home/greg/dev/labs/symfony-installer/vendor/symfony/console/Symfony/Compo
nent/Console/Application.php:889
    0.0217    1215500   6. Symfony\Installer\NewCommand->execute() /home/greg/dev/labs/symfony-installer/vendor/symfony/console/Symfony/Component/Cons
ole/Command/Command.php:252
    0.0217    1215656   7. Symfony\Installer\NewCommand->doExecute() /home/greg/dev/labs/symfony-installer/src/Symfony/Installer/NewCommand.php:50
    0.0221    1217700   8. Symfony\Installer\NewCommand->download() /home/greg/dev/labs/symfony-installer/src/Symfony/Installer/NewCommand.php:79
    0.0308    1708348   9. GuzzleHttp\Client->get() /home/greg/dev/labs/symfony-installer/src/Symfony/Installer/NewCommand.php:144
    0.0335    1860540  10. GuzzleHttp\Client->send() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Client.php:156
    0.0337    1867720  11. GuzzleHttp\Adapter\StreamingProxyAdapter->send() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Client.
php:193
    0.0337    1867880  12. GuzzleHttp\Adapter\Curl\CurlAdapter->send() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Adapter/Str$
amingProxyAdapter.php:34
    0.1241    2031052  13. GuzzleHttp\Event\RequestEvents::emitComplete() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Adapter/C
url/CurlAdapter.php:97
    0.1250    2043472  14. GuzzleHttp\Event\Emitter->emit() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Event/RequestEvents.php
:76
    0.1251    2043836  15. GuzzleHttp\Subscriber\Redirect->onComplete() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Event/Emitt
er.php:105
    0.1257    2046644  16. GuzzleHttp\Client->send() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Subscriber/Redirect.php:88
    0.1257    2046716  17. GuzzleHttp\Adapter\StreamingProxyAdapter->send() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Client.
php:193
    0.1258    2046716  18. GuzzleHttp\Adapter\Curl\CurlAdapter->send() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Adapter/Stre
amingProxyAdapter.php:34
    0.1266    2047240  19. curl_exec() /home/greg/dev/labs/symfony-installer/vendor/guzzlehttp/guzzle/src/Adapter/Curl/CurlAdapter.php:86

                                           
  [GuzzleHttp\Exception\RequestException]  

```

I added a php comment