Prioritize `jsxhint` over `jshint`

`jsxhint` depends on having `jshint` installed, so if I understand correctly, without this change `jsxhint` will never get selected. If you have `jsxhint` installed it's reasonable to assume you want to use it over `jshint` because it will work fine on normal `.js` file as well as `.jsx` file.

Ideally `jsxhint` would only be available if the file is an actual `jsx` file (i.e., has the `.jsx` extension or the `@jsx` pragma at the start of the file), but I am not sure if Syntastic can make the decision based on file extension or contents.