Update project badges in the readme
Update project badges in the readme

* Add a badge for the current RubyGems release
* Update the Code Climate badge to display the project's current GPA
* Size all badges consistently
* Use reference-style Markdown links

pepijnve/htty,shekkbuilder/htty,lanastasov/htty,ilesan-ypg/htty,shaunstanislaus/htty,bradparks/htty,yous/htty,paolomainardi/htty,Ribeiro/htty,dibenso/htty