Update README
Update README

Use a more specific version constraint in the Gemfile entry mentioned in the README

Fixes #472

BookBub/money-rails,antstorm/money-rails,RubyMoney/money-rails,BookBub/money-rails,BookBub/money-rails,antstorm/money-rails,antstorm/money-rails,RubyMoney/money-rails,RubyMoney/money-rails