Drop 500MB cap for 9.1

[headius]

For jruby/jruby#3739 we have decided to stop forcing a 500MB default heap max, since most current JVMs will choose their own, better, larger defaults. We've already removed it from the bash script for 9.1. We need to remove it from mjruby as well.

Unlike the bash script, we may also be able to do the memory size calculation with our 500MB in mind, if we want (as @enebo believes) to ensure we at least do not cause it to suddenly be lower on machines with less than 2GB (current Hotspot releases use 1/4*physical as default max). @enebo sent me a link to some C code to determine system memory (which I can't find now) that might serve as a good starting point for a "mruby-sysinfo" or something that provides some basic OS/platform information. From there we can choose an appropriate max or let JVM decide.

Or we can just let JVM decide, which is my preference :-) Either way, this is a 9.1 change, and 9.1 should ship mjruby.

[enebo]

@codefinger also points out another wrinkle with both our idea and about Java within docker itself:
http://matthewkwilliams.com/index.php/2016/03/17/docker-cgroups-memory-constraints-and-java-cautionary-tale/

Not sure what conclusion is to be made here but JVM ergonomics are broken because APIs are returning the wrong info to them...