Correcting Bundler.require in application.rb

We had deviated from the standard Rails 3.2.x
Bundler.require directive. When we upgraded
to the latest version of bootstrap-sass,
Publisher deploys started failing during
assets precompilation:

govuk_setenv publisher bundle exec rake \
RAILS_ENV=production RAILS_GROUPS=assets \
assets:precompile

@alext pointed this out, and suggested this
change to keep configs consistent with other
Rails 3.2.x apps.