Transform JRuby shebangs to absolute paths, fixing recursion within rbenv

The default shebang for binstubs in a JRuby installation is
`#!/usr/bin/env jruby`. On a rbenv-managed JRuby, this will execute the
`jruby` rbenv shim. In a situation where a JRuby tool like `gem` is
executed from within a running rbenv by a rbenv plugin, (e.g.
rbenv-gemset) this can trigger another rbenv run to resolve the shim.
This can lead to endless recursion of rbenv calls.

Fixes #471, closes #473
References jf/rbenv-gemset#56