Vim: update jsx detection

I just copied @wincent's jsx detection directly here. The 2 big
differences are:

1. Now that React 0.12 is out, we no longer need the React docblock
   pragma, so we no longer need to search for jsx in the file. This
   removes that.
2. The way I was doing jasmine detection was a little different and left
   out the ScanFile test. This new version includes the ScanFile test so
   it only sets the filetype to javascript.jasmine.jsx if it actually
   uses React.