[growthforecast] Support `kitchen-docker` testing environment.

```
diff --git a/site-cookbooks/growthforecast/.kitchen.yml b/site-cookbooks/growthforecast/.kitchen.yml
index 7076c9f..881e455 100644
--- a/site-cookbooks/growthforecast/.kitchen.yml
+++ b/site-cookbooks/growthforecast/.kitchen.yml
@@ -1,6 +1,7 @@
 ---
 driver:
-  name: vagrant
+  name: docker
+  socket: <%= ENV['DOCKER_HOST'] %>

 provisioner:
   name: chef_solo
@@ -8,14 +9,13 @@ provisioner:
   data_bags_path: ../../data_bags
   roles_path: ../../roles

-
 platforms:
   - name: ubuntu-12.04
-    driver:
-      box: ubuntu-1204x64
+    driver_config:
+      image: kazu634:ubuntu1204
   - name: ubuntu-14.04
-    driver:
-      box: ubuntu-1404x64
+    driver_config:
+      image: kazu634:ubuntu1404

 suites:
   - name: default
```