Update vim-json to the recommended version

The original vim-json has been [superseded](http://www.vim.org/scripts/script.php?script_id=1945) by [elzr's much-improved version](https://github.com/elzr/vim-json).  Pardon if you've already examined that version and decided against it, but if you haven't it's a must-see.
Merge pull request #16 from binaryphile/patch-1

Update vim-json to the recommended version