Update package.json

I just bumped netpbm to version 1.0.0, after accepting a pull request for a bug fix from a user of your module.

I'm suggesting that you set your dependency to ~1.0.0, so that your module tracks bug fixes in the netpbm module but does not get any API changes or other likely bc breaks.

Thanks for using the netpbm module!
Merge pull request #3 from boutell/patch-1

Update package.json