Remove redundant syntax highlighting

Newer versions of vim (such as 7.3.923) syntax highlight Rackup and JSON
files correctly. In the interest of keeping the dotfiles slim and our
machines up-to-date, I think the solution is to remove these from
dotfiles and install a newer version of vim in thoughtbot/laptop:

thoughtbot/laptop#118

vimrc

@@ -43,8 +43,6 @@ augroup vimrcEx
 
   " Set syntax highlighting for specific file types
   autocmd BufRead,BufNewFile Appraisals set filetype=ruby
-  autocmd BufRead,BufNewFile config.ru set filetype=ruby
-  autocmd BufRead,BufNewFile *.json set filetype=javascript
   autocmd BufRead,BufNewFile *.md set filetype=markdown
 
   " Enable spellchecking for Markdown