Remove support for Rails 3/Rails 3.1 apps.

This is a follow up of the latest discussion at #124 and I'm opening this so it won't be left for dead in a branch.

The idea is to drop support for Rails 3 and ship both jQuery `1.11.x` and `2.1.x` inside the gem. I believe we can ship both JavaScript files inside `vendor/assets/javascripts` and everyone will be able to `//= require` either `jquery` or `jquery` on their apps without any hassle.

I'll also gonna need to update the rake task that download the latest jQuery version to support both 1.11.x and 2.1.x releases, since there isn't a specific URL to grab the latest jQuery 2 script like we do with `http://code.jquery.com/jquery.js` and friends. The expected workflow would be to update the `version.rb` **first** and then running the task to download all the assets for the specific versions.

/summon @JangoSteve