Raise on missing translations in development/test

Lessons learned from http://robots.thoughtbot.com/foolproof-i18n-setup-in-rails
Move Formulaic into Features module
Remove unnecessary FactoryGirl.find_definitions

This line causes a "duplicate factory definition" error when running `rake
db:setup`, which is run by `bin/setup`.
Prevent empty div when there are no flashes
Remove confusing line about Git

Fixes https://github.com/thoughtbot/suspenders/issues/352
Link to CONTRIBUTING.md in README
Pick random port when generating Rails app

We have lots of Suspenders-generated apps. We run them using Foreman.
Randomizing the port number can help us run multiple apps locally while avoiding
conflicts.

Convert `bin_setup` file to use ERb and interpolate `port_number` into template.