update readme with dependencies badge

Author: Ivan Evgrafov

README.rdoc

@@ -6,6 +6,8 @@ This software has been developed as a product in Google Summer of Code 2015 (GSo
 
 {<img src="https://badge.fury.io/rb/gnuplotrb.svg" alt="Gem Version" />}[https://rubygems.org/gems/gnuplotrb]
 
+{<img src="https://gemnasium.com/dilcom/gnuplotrb.svg" alt="Dependency Status" />}[https://gemnasium.com/dilcom/gnuplotrb]
+
 {<img src="https://travis-ci.org/dilcom/gnuplotrb.svg?branch=master" alt="Build Status" />}[https://travis-ci.org/dilcom/gnuplotrb]
 {<img src="https://codeclimate.com/github/dilcom/gnuplotrb/badges/gpa.svg" alt="Code quality" />}[https://codeclimate.com/github/dilcom/gnuplotrb]
 {<img src="https://codeclimate.com/github/dilcom/gnuplotrb/badges/coverage.svg" alt="Test coverage" />}[https://codeclimate.com/github/dilcom/gnuplotrb]