Automatically fetch latest ruby-build version with 24h cache

[exterm]

Fetches the newest ruby-build version whenever ruby-build is used, with a 24 hour cache.

I wasn't sure about the cache, but it seems wasteful to fetch the newest version every time.

Fixes #464

[exterm]

I was so unsure about the cache that I added a commit to remove it. We can merge this with or without cache. Maintainer's choice @Stratus3D

[runephilosof-abtion]

asdf-nodejs automatically fetches latest, so asdf-ruby should probably be structured similarly.

asdf-nodejs has a mention of a cache https://github.com/asdf-vm/asdf-nodejs/blob/master/lib/commands/command-nodebuild
But I don't think it is the same kind of cache that you had, however I only skimmed the code.

[exterm]

I was curious and dug into the asdf-nodejs code a bit. They are smarter on the fetches, don't re-fetch everything every time. I added a commit that should speed things up a bit. However as I'm not sure whether @Stratus3D is interested in merging something like this, I have not done extensive testing of this new commit yet.