update-18.sh fails on vagrant

[petre2dor]

it tries to add gradinar user to gpio group and it fails because there is no such group in vagrant

[rcursaru]

and should fail, since the group is specific to raspberry for hardware reasons. possible solutions:

• create the group on vagrant also (serves no purpose)
• do a check before making the change (sane way of doing things)
• change it to fail silently :D