TEMPORARY revert chef upgrade from habitat/plan.sh since we are testing knife-opc removal

this commit should be used in the 'upgrade chef and other gems' branch.
well, not this commit (the reversion), but the actual commit.

Signed-off-by: Lincoln Baker <sabaker@progress.com>

Author: lbakerchef

src/chef-server-ctl/habitat/plan.sh

@@ -84,7 +84,8 @@ do_install() {
 
   cat > Gemfile << EOF
 source 'https://rubygems.org'
-gem 'chef', '~> 18.7.10'
+#gem 'chef', '~> 18.7.10'
+gem 'chef', '~>15.12.22'
 gem 'knife'
 EOF