No complete rewrite of package.json

This isn't as much of a Pull Request as it is a proposal. I really like grunt-bump but it always bothers me that it completely rewrites _package.json_. I have a certain layout and structure in that file and I'm a little bit of a Monk about it :)

I changed the `file.write` so that it only rewrites the version number and leaves the rest as is. This was a quick solution. Might not be the cleanest. What do you think?

Keep up the good work.