define an initializer rather than using config.before_initialize

the heroku_db_env railtie was building the database_configuration
*after* the 'active_record.initialize_database' which meant the database
connection was already established before the heroku_database.yaml could
be merged in. Secondly, the before_intialize code was running even when
config.assets.initialize_on_precompile was set to false was causing the precompile on slug compilation to break. Defining an initializer allows us to set the group as default which won't run on asset precompile