Moved sass-rails out of the assets group

Publisher deploys are failing because during
assets precompilation, we're seeing an issue
similar to:

twbs/bootstrap-sass#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:

rails/rails@49c4af4

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.

Gemfile

@@ -60,9 +60,11 @@ gem 'sidekiq-statsd', '0.1.2'
 # for sidekiq monitoring
 gem 'sinatra', '>= 1.3.0', :require => nil
 
+# NOTE: moved out of the assets group as a workaround for this issue:
+# https://github.com/twbs/bootstrap-sass/issues/523
+gem 'sass-rails', '3.2.6'
 group :assets do
   gem "therubyracer", "0.11.4"
-  gem 'sass-rails', '3.2.6'
   gem 'uglifier', '1.2.7'
 end