Trigger rubber:config after deploy:update_code

Also added logic to skip rubber:config and deploy:assets:precompile
during db role bootstrapping.

This commit doesn't break normal deploys (including deploy:migrations
or deploy:migrate) with a deploy.rb from old vulcanizations but
boostrap_db may fail if the assets task requires a database connection.
Re-vulcanizing to replace it with the new deploy.rb will fix this
though.
Merge pull request #277 from rubber/simpler_rubber_config_callback

Trigger rubber:config after deploy:update_code