Listen to .rb file changes only + Handle new and deleted files

This release introduces two changes:

Restrict changes to Ruby files only

Retest now only listens to ruby files is found. Retest feels faster too as we removed the massive Regex used to parse which files to listen to. It is now simpler and snappier.

This becomes the default until a better way to restrict unnecessary files from being watched is found.

Handle new and deleted files

Retest now listens to new and deleted file changes and behave accordingly without having to stop and stage your changes through git.