Moved sass-rails out of the assets group

Publisher deploys are failing because during assets precompilation, we're seeing an issue similar to:

https://github.com/twbs/bootstrap-sass/issues/523

This command fails during deploys:

govuk_setenv publisher bundle exec rake \
RAILS_ENV=production RAILS_GROUPS=assets \
assets:precompile

Rails 4 has removed the :assets group from the generated Gemfile. As mentioned in the above discussion, it seems OK for gems to move in that direction and to suggest removing :assets group:

https://github.com/rails/rails/commit/49c4af43ec5819d8f5c1a91f9b84296c927ce6e7

We have heavy weights like therubyracer in that group, which has been reported to be memory intensive. hence i'm avoiding removing :assets group entirely.

For the current issue, it is sufficient to move sass-rails out of the :assets group to get assets to precompile.