Update rails to 4.2.0

Update Rails to 4.2.0

Force suspenders to use its own bin/setup.up script
Update coffee-rails and sass-rails
Add web-console
Rails 4.2.0 sets `serve_static_files` to `ENV['RAILS_SERVE_STATIC_FILES'].present?` instead of `false`.

I tried to mimic the same behavior, but then I noticed that in commit `15d42ad3`, Dan Croak wrote:

````
We don't have to set `config.serve_static_assets = true` because
the `rails_12factor` gem does it for us.
````
But `rails_12factor` gem is installed only when suspenders in launched with `--heroku` option. So, shouldn't we keep the default rails configuration and remove it only when `--heroku` is passed to suspenders?