Added default path for 'artisan' based on a standard Laravel deployment.

Rocketeer tries to locate 'artisan' from the different search paths and, if it doesn't succeed, uses the fallback of <current_release>/artisan.
If the file is not there (as would happen in a --pretend run), prompt is still shown to allow the introduction of the correct path.

Originally it seemed to always expect 'artisan' path either explicitely or as a global installation.