remove gemnasium batch from README. [ci skip]

The rails/rails gemnasium page (https://gemnasium.com/rails/rails) is not helping much.
It lists 0 dependencies so there is not much to track. Also our gems mostly depend
on other gems we control. There is not much point in monitoring them.

The batch was introduced with rails#3909.

README.md

@@ -77,7 +77,6 @@ We encourage you to contribute to Ruby on Rails! Please check out the
 ## Code Status
 
 * [![Build Status](https://api.travis-ci.org/rails/rails.png)](https://travis-ci.org/rails/rails)
-* [![Dependencies](https://gemnasium.com/rails/rails.png?travis)](https://gemnasium.com/rails/rails)
 
 ## License