JRuby support

[headius]

JRuby has a few minor modifications to tmpdir that we should evaluate to merge or remove. Most of these are pretty simple changes.

https://gist.github.com/headius/f972fdb91565828c76e78dcf4ae7f5be

I will do the work to get these changes incorporated into the gem.

[hsbt]

I'm okay to merge JRuby's tmpdir changes if you want to keep it.

[headius]

So there are really only two changes:

• Switch the etc.so require to etc
• Deal with incompatibility in the change @nobu made in 2012: 82068d2

This second item will require fixes in JRuby, or reverting that line just for JRuby temporarily.

[headius]

See also jruby/jruby#3698 which describes the problem with using join instead of expand_path on JRuby.