chef-zero and berkshelf plugins conflict on json gem version, change order

vagrant-chef-zero depends on the json 1.7.7 gem, vagrant-berkshelf will bring in
a newer version if it installs first, breaking vagrant-chef-zero. This needs to
get fixed upstream too.

Change-Id: I7f9d40b6b759a33c66dbab8dc62261cc54788e9e