Upgrade PgHero, Rails, and Ruby. Use Alpine base image for size

[gaffneyc]

I ran into an error trying to run against PostgreSQL 10.3 due to changes in the stat functions. Upgrading both Rails and PgHero was pretty straight forward so I'm tackling both.

I removed rails_12factor as it's no longer necessary with Rails 5, though there are config changes that I've integrated into the config.

This upgrades Ruby to 2.5.0 (or whatever the latest 2.x series is) and switches to an Alpine based image.

Using the Debian base image produced a container that was ~850MB. Switching to the recommended Alpine image reduced that to ~250MB (3/10th the size).

Fixes #10