Reimplement fix for #233 on the develop branch.

Rocketeer tries to locate 'artisan' from the different search paths and, if it doesn't succeed, uses the fallback of /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.

This is same fix as #233 but on top of the 'develop' branch.