def refresh!
    raise 'source index not created from disk' if @spec_dirs.nil?
    load_gems_in(*@spec_dirs)
  end
#
Replaces the gems in the source index from specifications in the
directories this source index was created from.  Raises an exception if
this source index wasn't created from a directory (via from_gems_in or
from_installed_gems, or having spec_dirs set).