setting 'errno' to nil to prevent warnings fetching it

Dear Luis,

I know we don't maintain this gem anymore except for bug fixes, but this particular exception has a test case on ActiveRecord and it triggers a warning when trying to access `errno`. It would be great if this could be merged and a new gem released just so we wouldn't have to see the warning in the middle of the test run.

Unfortunately since `errno` is an instance var without "@" there's no other way i can think of to avoid the warning besides updating the gem. 

I know it's kind of frivolous, but do you think we could have the gem updated? It'd also close #25. If it's not possible I'll understand.

Thanks either way!