test suite should check compatibility with frozen string literals

related to #118

Author: flavorjones

concourse/tasks/rake-test/run.sh

@@ -4,6 +4,7 @@ set -e -x -u
 
 pushd loofah
 
+  export RUBYOPT="--enable-frozen-string-literal --debug=frozen-string-literal"
   bundle install
   bundle exec rake test