Vim: fix comment for `hidden` option

As @wincent pointed out, my comment here was inaccurate. So I read the
help on the subject and then took the comment that @wincent had used in
his Vim configuration.

Fixes #2
Vim: stop using JSXCS for JSCS syntastic JS checker

JSCS 1.8.0 adds support for using a custom esprima engine, and since
esprima-fb supports JSX, we can use that with a little configuration
instead of the JSXCS shim package.