Remove pre_packaging script from admin_ui package, store gems in blobstore.

Hey, all. 

I had a problems using pre_packaging script and wrote about them to [bosh-dev group](https://groups.google.com/a/cloudfoundry.org/forum/#!topic/bosh-dev/dKb5zpgh4Dc). That's why I decided to get rid of it.

In this PR I also needed to remove `config/blobs.yml` and `.final_builds` folder in order to create dev release and to test it. I've already told about this problems in #8. 

What do you think about this changes ?