Use only rails_stdout_logging, not rails_12factor

Rails 4.2.0 uses a `ENV['RAILS_SERVE_STATIC_FILES']` variable to determine by
default whether to server static files. We should use that community
convention rather than the `rails_serve_static_assets` gem, which is loaded by
`rails_12factor`. Removing that coupling also allows non-Heroku users to use
this convention.

When we remove our reliance on that gem, the remaining half of
`rails_12factor` that we still want to use in Heroku environments is the
`rails_stdout_logging` gem.

Extract a `serve_static_files_line` helper method as a way to reliably append
to the `config/environments/production.rb` file.

Use double quotes instead of single quotes on changed lines and the lines
around them.
Good catch, thanks.