Update FindRuby.cmake to also support Ruby 2.0 and 2.1.

The most significant change since Ruby 1.9 is the changed path of
config.h, from RbConfig::CONFIG['rubyhdrdir'] to
RbConfig::CONFIG['rubyarchhdrdir'].

Please consider applying this change also to the maint branch for
CMake 2.8.12.3.

The Debian Ruby maintainers are preparing to switch the default Ruby
version to 2.0, and have filed bugs against several packages using
CMake. It would be very helpfull if this change is included in the
next release to not require all packages to include a custom FindRuby
module.