ruby: update to 2.1.5

Ruby 2.1.5 has been released.

This release includes a security fix for a DoS vulnerability of REXML.
It is similar to the fixed vulnerability in the previous release, but
new and different from it.

CVE-2014-8090: Another Denial of Service XML Expansion
And, some bug fixes are also included. See tickets and ChangeLog for details.

Signed-off-by: Luiz Angelo Daros de Luca <luizluca@gmail.com>
Merge pull request #659 from luizluca/ruby-update

ruby: update to 2.1.5