Updated R download recipe to use URLTextSearcher

This allows the recipe to get either the 10.6 or 10.9 package by setting the OS_RELEASE to "snowleopard" or "mavericks". This recipe should be backwards compatible with existing overrides. If the OS_RELEASE is not set it defaults to the Snow Leopard version.
Added OS_RELEASE variable and removed minimum_os_version pkginfo key.