Move stdin argument to end of exec command, fixes #9

[nweddle]

Fixes #9

Looks like this was caused by the version of rubocop (0.49.1) included in ChefDK 2.0.13. The update to rubocop[1] has enforced proper usage of the --stdin flag:

-s/--stdin requires exactly one path.

This change moves stdin to the end of the exec command, to comply with the change in rubocop.

[1] rubocop/rubocop#4226

[mattstratton]

The only thing that doesn't make sense to me is that we never call rubocop directly; isn't it wrapped and pinned in cookstyle? If this fixes the chefdk issue, that's awesome, but I want to understand why.

[mattstratton]

I'm going to merge this in, and then I should be able to cut a new release later tonight (I just rebuilt my workstation and I don't know that I have all my apm stuff set up properly, heh)