update rails_template.rb for rails 4.x

remove the "public/index.html" file only if it exist, rails 4.x don't have this file.

spec/support/rails_template.rb

@@ -95,7 +95,7 @@ def set_id
 generate 'active_admin:install'
 
 inject_into_file "config/routes.rb", "\n  root to: redirect('/admin')", after: /.*::Application.routes.draw do/
-remove_file "public/index.html"
+remove_file "public/index.html" if File.exists? "public/index.html"
 
 # Devise master doesn't set up its secret key on Rails 4.1
 # https://github.com/plataformatec/devise/issues/2554