v0.6.0

This is a breaking change, though only minorly so for common use-cases.

• Removed Use#version and replaced it with Use#ref to allow referencing actions by a) complex version strings (e.g. 4.0.0) and b) by arbitrary branch names (e.g. master). It is most certainly still best practice to only reference tags within the ref field, but you're free to do whatever is necessar.