Monkey patch Exception#set_backtrace

Fix for #17.
I think that we can do something like [`activesupport/lib/active_support/core_ext/kernel/debugger.rb`](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/kernel/debugger.rb#L4-L8) instead of this. We can port it more easily to rails itself that way.

For now it's fine, though. I'll open a ticket and we'll revisit it.