We need to skip unit for now

There is a bug in the ChefDK release installed on the build nodes that
causes an exception when running the unit tests. Reference:

- chef-boneyard/chef-dk#411
- rubygems/rubygems#1287

We think we can run unit phase again after ChefDK 0.7.0 (final
release) is installed on the build nodes.

.delivery/config.json

@@ -12,6 +12,7 @@
     "publish",
     "quality",
     "security",
-    "smoke"
+    "smoke",
+    "unit"
   ]
 }