Fix chef cookbooks to work with ubuntu 14.04

The primary issue was that libshadow-ruby1.8 is not
present in 14.04. The secondary one is that default
ruby in 14.04 is 1.9, so update-alternatives step
fails.

Fix:
* drop the requirement of libshadow-ruby1.8, since
we set 1.9 by default.
* see if 'ruby' points to 1.9 and update symlinks only
in case it doesn't