We symlink Rubinius' `PREFIX/gems/bin` into `PREFIX/bin` so that new
RubyGems binstubs get added to the main bin directory and therefore
become available to rbenv.

However, by throwing away `irb`, `rake`, `rdoc`, and `ri` binstubs
during this process (which are non-executable and have an invalid
shebang), we break the same commands in latest versions of Rubinius.

This change ensures that these binstubs get preserved, their shebang
corrected to `#!PREFIX/bin/rbx`, their executable bit flipped on, and
copied over to the main bin directory.

Fixes #468