Improve jruby detection

[aburgel]

I'm seeing an issue with 3.2.0 on jruby, my builds are failing because bundler is attempting to install the ruby version of rbtree3, not the jruby version.

I think the current method of detecting jruby doesn't work in all cases, for example when using rvm on travis.

/home/travis/.rvm/gems/jruby-9.1.9.0/extensions/universal-java-1.8/2.3.0/rbtree3-0.5.0/mkmf.log
extconf failed, exit code 1
Gem files will remain installed in
/home/travis/.rvm/gems/jruby-9.1.9.0/gems/rbtree3-0.5.0 for inspection.
Results logged to
/home/travis/.rvm/gems/jruby-9.1.9.0/extensions/universal-java-1.8/2.3.0/rbtree3-0.5.0/gem_make.out
An error occurred while installing rbtree3 (0.5.0), and Bundler cannot
continue.
Make sure that `gem install rbtree3 -v '0.5.0' --source 'https://rubygems.org/'`
succeeds before bundling.
In rails_5_0.gemfile:
  mygem was resolved to 0.0.2, which depends on
    delayed-plugins-airbrake was resolved to 1.1.0, which depends on
      airbrake was resolved to 8.0.1, which depends on
        airbrake-ruby was resolved to 3.2.0, which depends on
          rbtree3

You can see that its trying to use rbtree3, not rbtree-jruby, despite being on jruby.

[kyrylo]

Weird that our CI didn't catch that. The line you're fixing was my 2nd attempt after the CI failed, so I thought we were good. Either way, thanks!