Speed up JRuby tests by improving the startup time

Emulate `jruby --dev` which improves JRuby's startup time.

See
https://github.com/jruby/jruby/wiki/Improving-startup-time#use-the---dev-flag

Follow @headius' advice and give Java more memory to reduce GC time.
@robin850 It seems @matthewd already fixed this for me while merging. Thanks :)