Exclude source assets from final package

app/assets, ee/app/assets, and vendor/assets are source directories used
by the rails asset pipeline, the compiled outputs in `public/assets` are
all that we need to ship to an end-user

Should save ~23Mb based on gitlab-ee@7732acd

    $ du -shc app/assets ee/app/assets vendor/assets
     17M    app/assets
    2.5M    ee/app/assets
    3.3M    vendor/assets
     23M    total

http://guides.rubyonrails.org/asset_pipeline.html
https://gitlab.com/gitlab-org/gitlab-ee/blob/7732acd111e63dfb46075e684ae33e025b2f6f0c/config/application.rb#L116