README: Drop Gemnasium badge, service gone (#638)

README.rdoc

@@ -6,7 +6,6 @@
 {<img src="https://coveralls.io/repos/ctran/annotate_models/badge.svg?branch=develop" />}[https://coveralls.io/r/ctran/annotate_models?branch=develop]
 {<img src="https://codeclimate.com/github/ctran/annotate_models/badges/gpa.svg" />}[https://codeclimate.com/github/ctran/annotate_models]
 {<img src="http://inch-ci.org/github/ctran/annotate_models.svg?branch=develop" alt="Inline docs" />}[http://inch-ci.org/github/ctran/annotate_models]
-{<img src="https://gemnasium.com/ctran/annotate_models.png" />}[https://gemnasium.com/ctran/annotate_models]
 
 Add a comment summarizing the current schema to the top or bottom of each of
 your...