Patch libyaml for CVE-2013-6393

CVE-2013-6393 is a buffer overflow vulnerability that affects libyaml. A quote
from a synopsis of the vulnerability states that:

  LibYAML is prone to a remote heap-based buffer-overflow vulnerability because
  it fails to properly sanitize user-supplied input.

  Successful exploits allow remote attackers to execute arbitrary code in the
  context of the vulnerable application. Failed exploit attempts likely result
  in denial-of-service conditions.