Changed dependency for awesome_nested_set

While trying to run Rails4 and Spree together an error about spree
dependency on awesome_nested_set being 3.0.0.rc.1 causes bundle install
to fail. On investigating the awesome_nested_set repository, there is no
3.0.0.rc.1 version on xouhito's version.

However there is one on collectiveideas. Meaning the gem file from a
blog post is just wrong so this is unneccesary.