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.