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.