Make `duckpan check` work properly.

- Changed 'unless' to 'if' because `check_requirements` returns `0` for success

Not entirely sure if `check_requirements` should return a `0` for success (it seems `exit` indicates success with 0 though) - but reversing this check was easier than reversing the logic and all other checks in DuckPAN.

//cc @jagtalon @mwmiller 
I agree with the formulation @nilnilnil has.  I would probably even add 
```perl 
# 0 indicates success
```
because I suck with DeMorgan's Laws sometimes.