Adds badges to README.md

Author: iMacTia

README.md

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 Faraday Middleware
 ==================
+[![Gem Version](https://badge.fury.io/rb/faraday_middleware.svg)](https://rubygems.org/gems/faraday_middleware)
+[![Build Status](https://travis-ci.org/lostisland/faraday_middleware.svg)](https://travis-ci.org/lostisland/faraday_middleware)
+[![Maintainability](https://api.codeclimate.com/v1/badges/a971ee5025b269c39d93/maintainability)](https://codeclimate.com/github/lostisland/faraday_middleware/maintainability)
 
 A collection of useful [Faraday][] middleware. [See the documentation][docs].