Use JRuby's built-in JSR-223 implementation

We had our own, but it is no longer necessary, because JRuby now
provides its own, which will assuredly be better maintained than ours.

This avoids and fixes many problems with JRuby scripting support.
Greeting.rb: use global variables

All variables passed in via the bindings are global,
so we need to use the dollar sign when referring to them.
JRubyTest: fix JRuby unit tests

They need to use global variables (prepended with $) where appropriate.

Clearing the bindings is also very tricky with the JRuby script engine,
so let's remove that test for the time being. (I explored setting the
"org.jruby.embed.clear.variables" attribute to true and/or
"org.jruby.embed.localvariable.behavior" system property to "transient",
but then the variables are cleared immediately following evaluation of
the script, making them unretrievable from the bindings afterwards.)
Merge branch 'simplify'

This branch switches the JRuby script language to lean on JRuby's
built-in JSR-223 implementation, and fixes up the unit tests and
templates to work as expected.