chore(badge): add RubyGem version badge

RubyGem version badge added to README

README.md

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 [![Build Status](https://travis-ci.org/mattfreer/active_model_form_objects.png)](https://travis-ci.org/mattfreer/active_model_form_objects)
 [![Coverage Status](https://coveralls.io/repos/mattfreer/active_model_form_objects/badge.png?branch=master)](https://coveralls.io/r/mattfreer/active_model_form_objects?branch=master)
 
+[![Gem Version](https://badge.fury.io/rb/active_model_form_objects.svg)](http://badge.fury.io/rb/active_model_form_objects)
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 This Gem provides an `ActiveModel::FormObject` module that can be included into a PORO. Includes a Rails Generator for Form Objects.
 
 Form Objects are used to encapsulate operations which are triggered by a form submission. They are particularly useful when multiple Models need to be updated by a single form submission. A common example would be a signup form that results in the creation of both a Company and a User.