Log MRI bug with inability to rescue SignalException

MRI's own API documentation examples show the ability to rescue
SignalException to trap signals, however this fails on recent 1.9 and
2.0 builds.

The following code works fine on MRI 1.8 and Rubinius, but explodes on MRI 1.9 and 2.0:

```ruby
begin
  Process.kill('HUP',Process.pid)
rescue SignalException => e
  puts "received Exception #{e}"
end
```