Refactored a bit the code for showing the badge for a rubygem downloads count

1. added posibility to show the downloads count for the latest stable version 
  * i used this url http://img.shields.io/gem/dv/rails/stable.svg
  * the badge will look like this:  ``` [downloads | 81k stable version ] ```
2. Added posibility to extract this information for a specific version
  * i used this url http://img.shields.io/gem/dv/rails/4.1.0.svg
  * the badge will look like this:  ``` [downloads | 258k version 4.1.0 ] ```

Please let me know what you think. Maybe this will be useful