Follow up to #16613

Hello,

This pull request is a follow-up to #16613. It simply enables the `--dev` behavior on JRuby at the Rakefile level instead of `.travis.yml`.

Since we want this flag to be enabled anytime we are running the tests under JRuby, let's enable this at the Rakefile level so people get the performance boost on their local checkout.

Moreover, we avoid having to update this particular line anytime the option changes on the JRuby side.

The only drawback is that we have to define it in every Rakefile but there's no big deal, this is already the case for other options. The extra allocation of 1024Mb has been kept since this is not the case by default with the `--dev` flag and this has been added to reduce the GC time ; let me know if you want to remove it as well.

Have a nice day.