dolfin_convert script does not work in 2018.1.0.dev0

[Filip Gokstorp]

Just requires two easy fixes to work again:

1. change line 1 to python3 instead of python (this allows dolfin modules to be found)
2. remove unused import of the missing dolfin_utils.commands module in on line 36

Thanks for the great work!

[Jack Hale]

@Saintis It would be really helpful if you could contribute this as a patch via a pull request!