Add method current_ruby_version

In order to use current ruby version from ENV var or constant

core/lib/generators/refinery/cms/cms_generator.rb

@@ -85,7 +85,7 @@ def append_heroku_gems!
 
       append_file "Gemfile", %Q{
 # The Ruby version is specified here so that Heroku uses the right version.
-ruby #{ENV['RUBY_VERSION'].inspect}
+ruby #{current_ruby_version.inspect}
 
 # Gems that have been added for Heroku support
 group :production do
@@ -94,6 +94,10 @@ def append_heroku_gems!
 }.sub("{{production_gems}}", production_gems.join("\n  "))
     end
 
+    def current_ruby_version
+      ENV['RUBY_VERSION'].presence || RUBY_VERSION
+    end
+
     def bundle!
       run 'bundle install'
     end