Raise on missing translations in test

Although we're generating a separate `spec/support/i18n.rb` file for i18n
(4d30479c3750ad5ce708cbf03ab8a58d484f2e8a), we're
once again **not** failing specs when a translation is missing.
Add normalize-rails to application.css.scss
Generate rails_helper, not spec_helper

RSpec 3.x introduces a `spec/rails_helper.rb` file
which contains all dependencies necessary to run specs that need Rails.

https://www.relishapp.com/rspec/rspec-rails/docs/upgrade#default-helper-files

Achieves the goal of "Documenting Explicit Dependencies Through Tests":

http://robots.thoughtbot.com/document-explicit-dependencies-through-tests

* Move Rails-specific things to rails_helper.rb.
* Require spec_helper.rb from rails_helper.rb.
* Remove duplication across files.
* We do not need to require "spec_helper" manually.
  It is required for us in `.rspec`:

    --color
    --warnings
    --require spec_helper
Generate a Rails 4.1.6 app
v1.17.0
Improve generated bin/setup

* Make script idempotent.
* Fail fast with install messages and non-zero exit status.
Remove unnecessary check for directory existence

`mkdir -p` will no-op if the directory already exists.
Use `uncomment_lines` Thor helper
Ignore the entire /tmp directory

This ignores the generated `tmp/heroku_commands` file instead of just the
`tmp/dummy` app.
Chome considers port 6000 unsafe

See http://superuser.com/questions/188058/which-ports-are-considered-unsafe-on-chrome