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.