$ rake 2>&1 | grep 'iruby.*frozen'
/Users/zzz/src/github.com/SciRuby/iruby/lib/iruby/formatter.rb:128: warning: literal string will be frozen in the future
/Users/zzz/src/github.com/SciRuby/iruby/lib/iruby/formatter.rb:96: warning: literal string will be frozen in the future
Before change:
After change: