Prevent ES6 feature use .jsx files from yielding jshint errors

Summary:
When I turned on the switch to transpile all JS files in webapp to ES6, I forgot
about this linter.

Without this change, if you try to use ES6 features in .js files (.jsx files are
special cased already), then you'll get lint errors.

Test Plan:
Add this to `javascript/shared-package/visit-tracking.js` in webapp.

    class Foo {
    }

Then run `arc lint`. You should see no errors.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D11535