Instead of setting the `JRUBY_OPTS` environment variable in all build jobs,
let's just set it on the jobs that need them.

Please note that this applies ONLY to CircleCI builds. On Travis, the
`JRUBY_OPTS` ENV variable will still be set for all builds. For detecting if a
job is running on JRuby on CircleCI, you can test if `ENV['CIRCLECI']` is `true`
*and* `ENV['JRUBY_OPTS']` is set.