JRuby 9.3 (head) and forward ships bundler

[enebo]

JRuby 9.3 will ship with bundler and jruby-head already does. This PR, like truffleruby, will not bother installing bundler since it is already installed.

I was confused about one thing. When JRuby 9.3 is released how do we change this action to say bundler is already installed? ruby-version appears to only report 'jruby' and 'jruby-head' to this JS function. I would like to make sure when we do put out that version we are not again reinstalling bundler.

[eregon]

Looks good, thanks.
When there is a 9.3 release we should update that logic to use something like the === 'ruby' logic for 9.3.

[eregon]

I added 9.3 to return true there too: 5ee1019

However this function alone doesn't decide whether to install Bundler or not.
It's a bit more complicated than that, I forgot: dddacd7
So for releases, we still install the latest Bundler release, which I think helps to get the latest Bundler fixes.

[enebo]

@eregon Thanks amazingly I had that in my original PR but I could not figure out how rubyVersion would get the 9.3. tbh part of me does not want to know :) Thanks for the additional tweak!