Update default ruby to 1.9.3-p484 (CVE-2013-4164)

See
https://www.ruby-lang.org/en/news/2013/11/22/ruby-1-9-3-p484-is-released/
Add a test for CVE-2013-4164. Will crash old Ruby!

If you are not on a recent version of Ruby, you will segfault.
Force Travis to Ruby 1.9.3-p484
Revert "Force Travis to Ruby 1.9.3-p484"

This reverts commit 25b0c86855f92101eb1c849f73573b2b48dc0838.
Revert CVE-2013-4164 test

This reverts commit 768821100916168f3782f72ad4dda60c299e5728.
Land #2674, Ruby 1.9.3-p484 (CVE-2013-4164)