* remove EOL ruby versions for security and maintenance reasons.
* only expose ruby_MAJOR_MINOR to the top-level. we don't provide
guarantees for the TINY version.
* mark all related packages as broken
* switch the default ruby version from 2.3.x to 2.4.x
Keep the `source` attrset distinct to prevent its entries from merging
with the top level attrs.
Since each type of source has a different set of entries for `source`,
this is the easiest way to keep them together.
This will pave the way for a new `url` type of source.
This is a mass-rebuild of many ruby packages,
notably those using `git` type sources.
