Support for Ruby 2.3

[etagwerker]

Context

In one of our projects, we could really use packwerk with Ruby 2.3.

Progress

Branch https://github.com/fastruby/packwerk/tree/v1.4.1 has made some progress but it still doesn't have a passing test suite.

[etagwerker]

I've made some progress with this but I still see many failures when running the test suite: https://gist.github.com/etagwerker/0ce39e24a793ee35ccbc01da950aef89

[etagwerker]

@SeanLF @adammathys this is a branch that is using sorbet: https://github.com/fastruby/packwerk/tree/v1.4.2

https://github.com/fastruby/packwerk/tree/v1.4.1 is an attempt to use packwerk without sorbet (probably a bad idea)