Attempts to reduce JRuby test flickering

It seems that the JAVA_OPTS we set cause our specs to periodically fail.
These options are important for cucumber runtime since rspec is spawned
many times, but are less important for the spec run since it is a single
ruby invokation.
Merge pull request #1232 from rspec/jruby_flickering

Attempts to reduce JRuby test flickering