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:

```sh
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.