Add php binary before build_bootstrap.php

build_bootstrap task should execute php binary, not the script itself.

```
DEBUG [bc4bfe9c]        /usr/bin/env: ./vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionBundle/Resources/bin/build_bootstrap.php
DEBUG [05f38581]        /usr/bin/env: ./vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionBundle/Resources/bin/build_bootstrap.php
cap aborted!
./vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionBundle/Resources/bin/build_bootstrap.php stdout: Nothing written
./vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionBundle/Resources/bin/build_bootstrap.php stderr: Nothing written
```

```
cd /var/www/foobar/releases/20140309213947 && ( SYMFONY_ENV=prod /usr/bin/env ./vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionB
undle/Resources/bin/build_bootstrap.php )
/usr/bin/env: ./vendor/sensio/distribution-bundle/Sensio/Bundle/DistributionBundle/Resources/bin/build_bootstrap.php: Permission denied
```