Adds a layout_partial_name method for page.

This returns a downcased and underscored version of the page layout.
Adds a more helpful `rake alchemy:install` task.

* It now asks for primary language (and defaults to English)
* It adds a generic element called article with helpful defaults and hints
* It redirects to sitemap after signing up the admin user

These should help new users to get Alchemy running even faster
Experimentally build against Rails 4.1.0.rc1
Do not fall back to bundler 1.5.1 while building on travis.

Bundler 1.5.3 should have fixed the issues.