Maintenance for JRuby 1.9.z release series

[jrgriffiniii]

The last release in the JRuby 1.9.z series (1.9.17.0) was released on April 23, 2018, and from this point onwards, there have been no successive releases in the JRuby 1.9.z series (https://www.jruby.org/2018/04/23/jruby-9-1-17-0.html).

Given that current attempts to update CircleCI builds fail for these releases (please see https://app.circleci.com/pipelines/github/samvera/rubydora/17/workflows/606dd16c-03fd-498c-8fd5-c04c29646361/jobs/193), I would like to please propose that support for the JRuby 1.9.z release series no longer be offered.