Fix skip_system_test not being preserved in update
Fix skip_system_test not being preserved in update

Previously running app:update always would template an application.rb
file without the `config.generators.system_tests = nil` set, even for
applications that previously had it set.

BlakeWilliams/rails,Erol/rails,yahonda/rails,esparta/rails,rails/rails,georgeclaghorn/rails,BlakeWilliams/rails,yahonda/rails,yawboakye/rails,georgeclaghorn/rails,Erol/rails,yahonda/rails,BlakeWilliams/rails,georgeclaghorn/rails,MSP-Greg/rails,esparta/rails,Erol/rails,georgeclaghorn/rails,MSP-Greg/rails,MSP-Greg/rails,MSP-Greg/rails,BlakeWilliams/rails,esparta/rails,yahonda/rails,rails/rails,yawboakye/rails,yawboakye/rails,Erol/rails,rails/rails,esparta/rails,yawboakye/rails,rails/rails