[close #159] Remove deprecated asset behavior

Rails3 deploys that do not successfully run `assets:precompile` will now fail.
The default cache skips installing nokogiri

Not sure how this wasn't caught earlier but with the default cache, nokogiri doesn't get installed using Ruby 2.0.0. We may want to change how we're testing for this, but for now this spec fix should keep the suite working for now.

Here is the new output:

```
-----> Fetching custom git buildpack... done
-----> Ruby app detected
-----> Using Ruby version: ruby-2.0.0
-----> Installing dependencies using Bundler version 1.3.2
       New app detected loading default bundler cache
       Running: bundle install --without development:test --path vendor/bundle --binstubs vendor/bundle/bin --deployment
       Using mini_portile (0.5.1)
       Using nokogiri (1.6.0)
       Using bundler (1.3.2)
       Your bundle is complete! It was installed into ./vendor/bundle
       Bundle completed (0.53s)
       Cleaning up the bundler cache.
       Removing sass (3.2.10)
       Removing railties (4.0.0)
       Removing rails_serve_static_assets (0.0.1)
       Removing activerecord-deprecated_finders (1.0.3)
       Removing polyglot (0.3.3)
```
Merge pull request #161 from heroku/schneems/remove-runtime-asset-compilation

[close #159] Remove deprecated asset behavior