Refresh min ruby version for this repo

[jaymzh]

This repo (mixlib-install), is the bootstrap mechanism for folks to get a modern build of ruby from a system ruby which may be old, and may be even an unsupported version. So, we need to look at all supported platforms, determine the oldest system ruby, and then bump our min version to that (or, that may mean keeping it where it is), then fixing our rubygem dependencies to accommodate that version.