Updated bootstrap version to alpha 6

[jeremylynch]

Bootstrap alpha 6 has been released. Thought I would update this.

[DanielKehoe]

I'm sorry but there's much more to do to update Rails Composer to Bootstrap 4 Alpha 6. The new version has many changes to the grid and navbar. The rails_layout gem is used by Rails Composer to generate pages with suitable navbar and grid. See RailsApps/rails_layout@8e101af
for the initial implementation for Bootstrap 4 Alpha 3. Can you make the changes needed for the rails_layout gem?

[jeremylynch]

Thanks @DanielKehoe,

I've started working on this. What is the forms.css.scss used for? It isn't being generated in my projects and it seems to have a lot of the same content as the overrides css file.

Thanks

[jeremylynch]

I've created a PR over at rails_layout