Use absolute require for spec_helper instead of relative

Rspec add /spec to the path, so we can require 'subdir/spec_helper' to load /spec/subdir/spec_helper.
See http://stackoverflow.com/questions/5061179/how-to-load-a-spec-helper-rb-automatically-in-rspec-2
Simplify specs

Remove the useless unit directory.
Add a support method to find bash resources with chefspec.
Indicated the cookbook only supports ubuntu.
Apt-get update with options to only reload the local repository.