Include VERSION file when copying cookbooks

"The Berkshelf way" defines cookbook versions in a VERSION file.
This makes the metadata.rb file contain:

    IO.read(File.join(File.dirname(__FILE__), 'VERSION'))

By default, Berkshelf adds a "rescue '0.1.0'", but this feels wrong
and also makes it impossible to do any kind of "version checking".
Loosen Celluloid version restriction to work with Berkshelf 2