[wordpress] Do not run tests when `wordpress` cookbook is modified.

```
diff --git a/tasks/tests/build-images.rake b/tasks/tests/build-images.rake
index 38126c4..44d9f76 100755
--- a/tasks/tests/build-images.rake
+++ b/tasks/tests/build-images.rake
@@ -5,7 +5,7 @@ require 'rake'
 desc "Build docker images"
 task :build do
   # Retrieve the list of the modified files:
-  modified_files = `git diff --name-only master..\`git symbolic-ref HEAD\``
+  modified_files = `git diff --name-only master..\`git symbolic-ref HEAD\` | grep -v wordpress`

   # If no cookbooks are modified, exit task:
   unless modified_files.include?("site-cookbooks")
```
Merge pull request #160 from kazu634/wordpress-supports-kitchen-docker

[wordpress] `Wordpress` cookbook supports `kitchen-docker` testing environment.