[base] Do not specify path/to/docker.

As Circle CI does not place `docker` binary in the same location on our machine,
do *not* specify the `path/to/docker`.

```
diff --git a/site-cookbooks/base/.kitchen.yml b/site-cookbooks/base/.kitchen.yml
index 8e12795..493c752 100644
--- a/site-cookbooks/base/.kitchen.yml
+++ b/site-cookbooks/base/.kitchen.yml
@@ -1,7 +1,6 @@
 ---
 driver:
   name: docker
-  binary: /usr/local/bin/docker
   socket: <%= ENV['DOCKER_HOST'] %>

 provisioner:
```