Ship both jQuery 1 and jQuery 2.

A follow up for the discussions @ #124 and #157. Now apps with `jquery-rails` 4 can either `//= require jquery` for including the jQuery 1.x inside an existing manifest or `//=require jquery2` to bring jQuery 2.1.

For maintainers who want to update the vendored scripts, we now need to update the `version.rb` file **before** running the `update_jquery` task, as it looks like there isn't an URL for the latest jQuery 2.x assets.

* [x] Update the rake task.
* [x] Update `README` to mention jQuery 2
* [x] Update the `VERSIONS` file.

/cc @JangoSteve @rafaelfranca