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:

https://github.com/thoughtbot/laptop/pull/118
Adds local configuration file for vim bundles