Added definition for 1.8.7-p374-cheerful0 [fixes CVE-2013-4164], closes #456.

This build is an unofficial backport of the patch for
"Heap Overflow in Floating Point Parsing (CVE-2013-4164)"
to Ruby 1.8.7-p374.

This patch was provided by Makandra and is based on the offical
patches for Ruby 1.9 and Ruby 2:
http://makandracards.com/railslts/19977-backported-fix-for-heap-overflow-in-floating-point-parsing-cve-2013-4164

To indicate the unofficial nature of this version, "cheerful0"
was added to the version string, and the version date was updated:

```
$ ruby -v
ruby 1.8.7 (2013-11-25 patchlevel cheerful0 374) [i686-darwin13.0.0]
```