Reimplemented ruby module to use distro packages

Use of rbenv as the underlying manager of Ruby versions results in very
long initial provisioning times since ruby-build must compile each new
version (it does not support binary packages). Furthermore, rbenv's use
of shims leads to situations where gem's can appear to be installed but
fail upon execution (possibly related to bug 66764).

The ruby module has been refactored to use system packages for versions
of Ruby and RubyGems, but maintains isolation of gem dependencies using
the newest version of Bundler installed from gems and executed with the
correct Ruby version (avoiding regression of fix to bug 50993).

Bug: 66763
Change-Id: I1808e33d9c79f75bb1ae22609b4ba5422a1baff4