Bug#926280: Don't bundle rubygems

[Moritz Muehlenhoff]

Package: jruby
Severity: important

(This bug isn't really actionable yet, as it depends on #926278 getting fixed
in src:ruby2.5)

Please don't use the bundled rubygems any longer, but instead a copy shared
with the C-based Ruby interpreter.

Given that most of the security issues in the C-based interpreter don't
affect Jruby (apart from the rubygems) this will considerably reduce the
overhead for keeping jruby updated in stable/oldstable.

I spoke to upstream (CCed) earlier and they confirmed that jruby bundles
the rubygems unmodified, so that should not cause any run time issues.

Cheers,
        Moritz