Adding Dockerfile for `ruby 2.1` and updating README files

# Dockerfiles

The repository contains Dockerfiles for various public Docker images, namely:
* `azukiapp/php-apache:5.6`:
  - Apache2
  - PHP Version 5.5.9
  - Composer

* `azukiapp/php-apache:5.5`:
  - Apache2
  - PHP Version 5.6.3
  - Composer

* `azukiapp/ruby:2.1`
  - Ruby 2.1.2
  - RubyGems 2.3.0
  - Bundler
  - ImageMagick

* All images contains:
  - Git
  - MySQL Client
  - PostgreSQL Client
  - MongoDB


## License

Dockerfiles distributed under the [Apache License](LICENSE).