[common] Create `Dockerfile` for Ubuntu.

- Create `Dockerfile` for Ubuntu 12.04.
- Create `Dockerfile` for Ubuntu 14.04.
[common] Add task for `docker build` images.
[common] Specify `docker/*.tar` for `git` to ignore.

```
diff --git a/.gitignore b/.gitignore
index 9b8fa9b..ef3c5a3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,3 +9,4 @@ test/spec/reports
 test/Rakefile
 *.swp
 .bundle
+docker/*.tar
```
[common] Add `test` rake task.

```
diff --git a/tasks/tests/ci.rake b/tasks/tests/ci.rake
new file mode 100755
index 0000000..a612849
--- /dev/null
+++ b/tasks/tests/ci.rake
@@ -0,0 +1,12 @@
+#!/usr/bin/env rake
+
+require 'rake'
+
+desc "Run CI tests."
+task :test do
+  cookbooks = `git diff --name-only master..$(git symbolic-ref HEAD) | grep site-cookbooks | cut -f 2 -d / | awk '!a[$0]++'`
+
+  cookbooks.split('\n').each do |cookbook|
+    puts "cd #{PROJ_ROOT}/site-cookbook/#{cookbook}"
+  end
+end
```
[common] Modify `circle.yml` to execute tests.

- Execute tests
- Download `.chef_databag_secret` to the appropriate location
- Cache `cookbooks` directory
[common] Specify the `Ruby` version.

```
diff --git a/.ruby-version b/.ruby-version
new file mode 100644
index 0000000..ac2cdeb
--- /dev/null
+++ b/.ruby-version
@@ -0,0 +1 @@
+2.1.3
```
[base] Modify `.kitchen.yml` to support `docker` testing.
[base] Modify test, due to the timezone config difference between Japanese and US. version of Ubuntu.
[base] Do not generate locale configuration, under `Circle CI` container.

Since `Circle CI` does not support locale modification in `docker` container,
do not generate locale configuration under `Circle CI` container.