@thomasw, @blaix 
NPM and Bower both use a 2-space indent when they write/overwrite their json files. Which is probably why this one has a 2-space indent.

Is it okay if we allow 2-space indents, just for `package.json` and `bower.json`?

Otherwise we're going to have to keep spotting and correcting the indents by hand every time we add a dependency via `--save` or `--save-dev`.