should gracefully  fail if required commands are not found

Travis seems to be missing the setfacl command, so running `composer install` fails

Here is the build output:
```
Script Umpirsky\PermissionsHandler\ScriptHandler::setPermissions handling the post-install-cmd event terminated with an exception

[Symfony\Component\Process\Exception\ProcessFailedException]

The command "setfacl -R -m u:"":rwX -m u:`whoami`:rwX var/cache" failed.

Exit Code: 127(Command not found)

Output:

================

Error Output:

================

sh: 1: setfacl: not found 
``