Bump to ruby-1.9.3-p550 (CVE-2014-8080)

Avoid the DoS vulnerability described at:

https://www.ruby-lang.org/en/news/2014/10/27/rexml-dos-cve-2014-8080/

Landing this PR will make rvm complain until users actually install p550, and there are no rvm binaries for this, so it'll need to be compiled locally. Then, users will need to re-bundle install.

So, it'll take a while for people to make the switch. 10-15 minutes?

I expect no issues with this change, of course, but I'm installing ruby right now.