README.md: Added version badge (rack#199)

* README.md: Added more badges

Sorry, once again a branch in this repo: it's just so convenient to edit straight in the GitHub UI. I promise to delete the branch afterwards. 😎

* Removed download count

...after peer review.

README.md

@@ -1,4 +1,5 @@
 # Rack::Test
+[![Gem Version](https://badge.fury.io/rb/rack-test.svg)](https://badge.fury.io/rb/rack-test)
 [<img src="https://travis-ci.org/rack-test/rack-test.svg?branch=master" />](https://travis-ci.org/rack-test/rack-test)
 [![Code Climate](https://codeclimate.com/github/codeclimate/codeclimate/badges/gpa.svg)](https://codeclimate.com/github/codeclimate/codeclimate)
 [![Test Coverage](https://codeclimate.com/github/codeclimate/codeclimate/badges/coverage.svg)](https://codeclimate.com/github/codeclimate/codeclimate/coverage)