Update to Slim Skeleton App

I made some updates to the Slim skeleton app I put together last year. It still uses my fork of Slim and I've pulled in all the latest changes from the original repository (v2.4.2). All tests passing. FWIW, I migrated an application this week without any breakages.

The skeleton itself now uses Slim's Views repository, replacing the deprecated Extras.

Using Composer to generate an app is straightforward enough:

$ composer create-project moz-morris/slim-skeleton [app-name]