remove nodejs as a requirement for production

[jordimassaguerpla]

Assets are not being recompiled in production for Rails 4

http://guides.rubyonrails.org/asset_pipeline.html

Thus, we don't need any javascript engine in production (nor the gems
that requires that)

Let's remove nodejs and the gems that require that from production
(=assets gems)

However, we do need nodejs and these gems for precompiling the assets,
thus we use an env variable for enabling/disabling them.

This way we can do:

For precompiling assets in the spec file:

PACKAGING=yes IGNORE_ASSETS=no RAILS_ENV=production bundle exec rake
assets:precompile

and then, for bundling the gems:

PACKAGING=yes IGNORE_ASSETS=yes bundle install

[flavio]

You can simply write:

if ENV["IGNORE_ASSETS"] == "yes"

[jordimassaguerpla]

@flavio: I've fixed the mentioned issues