JRuby slow times out travis builds

[myronmarston]

The JRuby build has been intermittently timing out the travis build at 50 minutes. However, it used to run much faster. As recently as 3 months ago, it was taking < 15 minutes:

https://travis-ci.org/rspec/rspec-core/builds/5360425

We should figure out what changed and how to make it faster.

[JonRowe]

FYI.

This is the last short build: https://travis-ci.org/rspec/rspec-core/jobs/5360431
and this is the first long build: https://travis-ci.org/rspec/rspec-core/jobs/5432130

Commits:
(short) e6714c0
(long) 63ebcc6

[JonRowe]

Figured it out, see #957, before we were running the JVM in 32bit client mode for quicker startup time, but then Java was upgraded on travis to a version that doesn't support that mode, so we dropped the option. A bit of digging found the relevant steps to restore something close to client mode on the current Java and thus the build time seems to have dropped to ~15minutes again.

I'll hold the PR open to give you guys time to review then merge and pluck the commit across to 2-14 and 2-99 too ;)

/cc @alindeman @samphippen @soulcutter