Support for specifying env in assets.yml

Similar to my buildpack PR: https://github.com/cloudfoundry/vcap-yeti/pull/9

Our cf installation doesn't have access to the internet.  So in order to install all of the ruby apps that test the non ruby aspects of CF I need to be able to specify a proxy for bundle install when running tests.

Here is an example usage.  This asset config allows me to deploy ruby apps without the default ruby build pack installed and without direct internet access from my DEAs:
```
simple_app:
  memory: 512
  command: "bundle exec ruby foo.rb -p $VCAP_APP_PORT"
  path: "assets/sinatra/hello_vcap"
  buildpack: https://github.com/cloudfoundry/heroku-buildpack-ruby.git
  env:
    http_proxy: http://someproxy:80
    https_proxy: http://someproxy:80
```