Add definition for heroku-patched 1.8.7-p375

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.

The definition is called `heroku-1.8.7-p375`.

The defnition refers to a tarball of this tag:
https://github.com/heroku/ruby/releases/tag/v1_8_7_375